Mailing List Archive: [PATCH 5/9] ARM: Xen: fix initial build problems:

[PATCH 5/9] ARM: Xen: fix initial build problems:

Oct 9, 2012, 8:22 AM

Post #1 of 17 (452 views)

* The XEN_BALLOON code requires the balloon infrastructure that is not
getting built on ARM.

* The tmem hypercall is not available on ARM

* ARMv6 does not support cmpxchg on 16-bit words that are used in the

* sys-hypervisor.c needs to include linux/err.h in order to use the
IS_ERR/PTR_ERR/ERR_PTR family of functions.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>
Cc: xen-devel@lists.xensource.com
---
arch/arm/Kconfig | 1 +
drivers/xen/Kconfig | 2 ++
drivers/xen/sys-hypervisor.c | 1 +
3 files changed, 4 insertions(+)

diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 6d2f7f5..85eaac3 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -1846,6 +1846,7 @@ config XEN_DOM0
config XEN
bool "Xen guest support on ARM (EXPERIMENTAL)"
depends on EXPERIMENTAL && ARM && OF
+ depends on !CPU_V6
help
Say Y if you want to run Linux in a Virtual Machine on Xen on ARM.

diff --git a/drivers/xen/Kconfig b/drivers/xen/Kconfig
index d4dffcd..126d8ce 100644
--- a/drivers/xen/Kconfig
+++ b/drivers/xen/Kconfig
@@ -3,6 +3,7 @@ menu "Xen driver support"

config XEN_BALLOON
bool "Xen memory balloon driver"
+ depends on !ARM
default y
help
The balloon driver allows the Xen domain to request more memory from
@@ -145,6 +146,7 @@ config SWIOTLB_XEN

config XEN_TMEM
bool
+ depends on !ARM
default y if (CLEANCACHE || FRONTSWAP)
help
Shim to interface in-kernel Transcendent Memory hooks
diff --git a/drivers/xen/sys-hypervisor.c b/drivers/xen/sys-hypervisor.c
index 5e5ad7e..66a0a14 100644
--- a/drivers/xen/sys-hypervisor.c
+++ b/drivers/xen/sys-hypervisor.c
@@ -11,6 +11,7 @@
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/kobject.h>
+#include <linux/err.h>

#include <asm/xen/hypervisor.h>
#include <asm/xen/hypercall.h>
--
1.7.10

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Re: [PATCH 5/9] ARM: Xen: fix initial build problems: [ In reply to ]

Ian.Campbell at citrix

Oct 9, 2012, 8:35 AM

Post #2 of 17 (448 views)

Permalink

On Tue, 2012-10-09 at 16:22 +0100, Arnd Bergmann wrote:
> * The XEN_BALLOON code requires the balloon infrastructure that is not
> getting built on ARM.

I've got patches to enable this, but not for 3.7 so this looks good for
now.

> * The tmem hypercall is not available on ARM
>
> * ARMv6 does not support cmpxchg on 16-bit words that are used in the

missing the end of this sentence?

> * sys-hypervisor.c needs to include linux/err.h in order to use the
> IS_ERR/PTR_ERR/ERR_PTR family of functions.

I tripped over this too. Not sure where that patch got to though.

> Signed-off-by: Arnd Bergmann <arnd@arndb.de>

Acked-by: Ian Campbell <ian.campbell@citrix.com>

> Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
> Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> Cc: Jeremy Fitzhardinge <jeremy@goop.org>
> Cc: xen-devel@lists.xensource.com
> ---
> arch/arm/Kconfig | 1 +
> drivers/xen/Kconfig | 2 ++
> drivers/xen/sys-hypervisor.c | 1 +
> 3 files changed, 4 insertions(+)
>
> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> index 6d2f7f5..85eaac3 100644
> --- a/arch/arm/Kconfig
> +++ b/arch/arm/Kconfig
> @@ -1846,6 +1846,7 @@ config XEN_DOM0
> config XEN
> bool "Xen guest support on ARM (EXPERIMENTAL)"
> depends on EXPERIMENTAL && ARM && OF
> + depends on !CPU_V6
> help
> Say Y if you want to run Linux in a Virtual Machine on Xen on ARM.
>
> diff --git a/drivers/xen/Kconfig b/drivers/xen/Kconfig
> index d4dffcd..126d8ce 100644
> --- a/drivers/xen/Kconfig
> +++ b/drivers/xen/Kconfig
> @@ -3,6 +3,7 @@ menu "Xen driver support"
>
> config XEN_BALLOON
> bool "Xen memory balloon driver"
> + depends on !ARM
> default y
> help
> The balloon driver allows the Xen domain to request more memory from
> @@ -145,6 +146,7 @@ config SWIOTLB_XEN
>
> config XEN_TMEM
> bool
> + depends on !ARM
> default y if (CLEANCACHE || FRONTSWAP)
> help
> Shim to interface in-kernel Transcendent Memory hooks
> diff --git a/drivers/xen/sys-hypervisor.c b/drivers/xen/sys-hypervisor.c
> index 5e5ad7e..66a0a14 100644
> --- a/drivers/xen/sys-hypervisor.c
> +++ b/drivers/xen/sys-hypervisor.c
> @@ -11,6 +11,7 @@
> #include <linux/kernel.h>
> #include <linux/module.h>
> #include <linux/kobject.h>
> +#include <linux/err.h>
>
> #include <asm/xen/hypervisor.h>
> #include <asm/xen/hypercall.h>

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Re: [PATCH 5/9] ARM: Xen: fix initial build problems: [ In reply to ]

stefano.stabellini at eu

Oct 9, 2012, 8:37 AM

Post #3 of 17 (446 views)

Permalink

Thanks for the patch and sorry for the build breakage!

On Tue, 9 Oct 2012, Arnd Bergmann wrote:
> * The XEN_BALLOON code requires the balloon infrastructure that is not
> getting built on ARM.
>
> * The tmem hypercall is not available on ARM
>
> * ARMv6 does not support cmpxchg on 16-bit words that are used in the
>
> * sys-hypervisor.c needs to include linux/err.h in order to use the
> IS_ERR/PTR_ERR/ERR_PTR family of functions.
>
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
> Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> Cc: Jeremy Fitzhardinge <jeremy@goop.org>
> Cc: xen-devel@lists.xensource.com
> ---
> arch/arm/Kconfig | 1 +
> drivers/xen/Kconfig | 2 ++
> drivers/xen/sys-hypervisor.c | 1 +
> 3 files changed, 4 insertions(+)
>
> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> index 6d2f7f5..85eaac3 100644
> --- a/arch/arm/Kconfig
> +++ b/arch/arm/Kconfig
> @@ -1846,6 +1846,7 @@ config XEN_DOM0
> config XEN
> bool "Xen guest support on ARM (EXPERIMENTAL)"
> depends on EXPERIMENTAL && ARM && OF
> + depends on !CPU_V6
> help
> Say Y if you want to run Linux in a Virtual Machine on Xen on ARM.

Considering that we rely on the virtualization extensions, this one can
be:

depends on CPU_V7

The rest looks fine. I can submit a second patch to change !CPU_V6 into
CPU_V7 later, if you prefer.

> diff --git a/drivers/xen/Kconfig b/drivers/xen/Kconfig
> index d4dffcd..126d8ce 100644
> --- a/drivers/xen/Kconfig
> +++ b/drivers/xen/Kconfig
> @@ -3,6 +3,7 @@ menu "Xen driver support"
>
> config XEN_BALLOON
> bool "Xen memory balloon driver"
> + depends on !ARM
> default y
> help
> The balloon driver allows the Xen domain to request more memory from
> @@ -145,6 +146,7 @@ config SWIOTLB_XEN
>
> config XEN_TMEM
> bool
> + depends on !ARM
> default y if (CLEANCACHE || FRONTSWAP)
> help
> Shim to interface in-kernel Transcendent Memory hooks
>
> diff --git a/drivers/xen/sys-hypervisor.c b/drivers/xen/sys-hypervisor.c
> index 5e5ad7e..66a0a14 100644
> --- a/drivers/xen/sys-hypervisor.c
> +++ b/drivers/xen/sys-hypervisor.c
> @@ -11,6 +11,7 @@
> #include <linux/kernel.h>
> #include <linux/module.h>
> #include <linux/kobject.h>
> +#include <linux/err.h>
>
> #include <asm/xen/hypervisor.h>
> #include <asm/xen/hypercall.h>
> --
> 1.7.10
>

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Re: [PATCH 5/9] ARM: Xen: fix initial build problems: [ In reply to ]

arnd at arndb

Oct 9, 2012, 8:39 AM

Post #4 of 17 (453 views)

Permalink

On Tuesday 09 October 2012, Ian Campbell wrote:
> > * The tmem hypercall is not available on ARM
> >
> > * ARMv6 does not support cmpxchg on 16-bit words that are used in the
>
> missing the end of this sentence?

Right, I meant to say

* ARMv6 does not support cmpxchg on 16-bit words that are used in the
Xen grant table code, so we must ensure that Xen support is only built
on ARMv7-only kernels not combined ARMv6/v7 kernels.

This should be fixed differently in the future.

Arnd

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Re: [PATCH 5/9] ARM: Xen: fix initial build problems: [ In reply to ]

linux at arm

Oct 9, 2012, 9:04 AM

Post #5 of 17 (443 views)

Permalink

On Tue, Oct 09, 2012 at 05:22:59PM +0200, Arnd Bergmann wrote:
> * The XEN_BALLOON code requires the balloon infrastructure that is not
> getting built on ARM.
>
> * The tmem hypercall is not available on ARM
>
> * ARMv6 does not support cmpxchg on 16-bit words that are used in the

"in the" what?

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Re: [PATCH 5/9] ARM: Xen: fix initial build problems: [ In reply to ]

Ian.Campbell at citrix

Oct 9, 2012, 9:10 AM

Post #6 of 17 (452 views)

Permalink

On Tue, 2012-10-09 at 16:39 +0100, Arnd Bergmann wrote:
> On Tuesday 09 October 2012, Ian Campbell wrote:
> > > * The tmem hypercall is not available on ARM
> > >
> > > * ARMv6 does not support cmpxchg on 16-bit words that are used in the
> >
> > missing the end of this sentence?
>
> Right, I meant to say
>
> * ARMv6 does not support cmpxchg on 16-bit words that are used in the
> Xen grant table code, so we must ensure that Xen support is only built
> on ARMv7-only kernels not combined ARMv6/v7 kernels.
>
> This should be fixed differently in the future.

Is this is a build time failure because gcc/gas/etc refuses to generate
these instructions if it is configured for v6?

I ask because if it is only a runtime issue then we can reason that if
we are running Xen specific grant table code, then we must be running on
Xen and therefore must necessarily be running on a v7 (because Xen only
support v7+virt extensions) even if the kernel happens to be capable of
running on v6 too.

Ian.

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Re: [PATCH 5/9] ARM: Xen: fix initial build problems: [ In reply to ]

arnd at arndb

Oct 9, 2012, 11:19 AM

Post #7 of 17 (442 views)

Permalink

On Tuesday 09 October 2012, Ian Campbell wrote:
> On Tue, 2012-10-09 at 16:39 +0100, Arnd Bergmann wrote:
> > On Tuesday 09 October 2012, Ian Campbell wrote:
> > > > * The tmem hypercall is not available on ARM
> > > >
> > > > * ARMv6 does not support cmpxchg on 16-bit words that are used in the
> > >
> > > missing the end of this sentence?
> >
> > Right, I meant to say
> >
> > * ARMv6 does not support cmpxchg on 16-bit words that are used in the
> > Xen grant table code, so we must ensure that Xen support is only built
> > on ARMv7-only kernels not combined ARMv6/v7 kernels.
> >
> > This should be fixed differently in the future.
>
> Is this is a build time failure because gcc/gas/etc refuses to generate
> these instructions if it is configured for v6?
>
> I ask because if it is only a runtime issue then we can reason that if
> we are running Xen specific grant table code, then we must be running on
> Xen and therefore must necessarily be running on a v7 (because Xen only
> support v7+virt extensions) even if the kernel happens to be capable of
> running on v6 too.

The underlying reason of course is that ARMv6 doesn't have those
instructions. The symptom you see is a link error because gcc emits
a reference to the (intentionally missing) __bad_cmpxchg() function
from

static inline unsigned long __cmpxchg(volatile void *ptr, unsigned long old,
unsigned long new, int size)
{
unsigned long oldval, res;

switch (size) {
#ifndef CONFIG_CPU_V6 /* min ARCH >= ARMv6K */
case 1:
...
break;
case 2:
...
break;
#endif
case 4:
...
break;
default:
__bad_cmpxchg(ptr, size);
oldval = 0;
}
return oldval;
}

The possible solutions I can see for this are:

* change the grant table format to use 32 bits for the flags on ARM
* change the code to always cmpxchg the entire 32 bit word including the flags.
* implement your own cmpxchg wrapper that may be implemented using a spinlock
rather than cmpxchg if ARMv6 is enabled.

Arnd

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Re: [PATCH 5/9] ARM: Xen: fix initial build problems: [ In reply to ]

arnd at arndb

Oct 9, 2012, 11:21 AM

Post #8 of 17 (440 views)

Permalink

On Tuesday 09 October 2012, Stefano Stabellini wrote:
> > config XEN
> > bool "Xen guest support on ARM (EXPERIMENTAL)"
> > depends on EXPERIMENTAL && ARM && OF
> > + depends on !CPU_V6
> > help
> > Say Y if you want to run Linux in a Virtual Machine on Xen on ARM.
>
> Considering that we rely on the virtualization extensions, this one can
> be:
>
> depends on CPU_V7
>
> The rest looks fine. I can submit a second patch to change !CPU_V6 into
> CPU_V7 later, if you prefer.

CPU_V6 and CPU_V7 are not exclusive, I saw the problem when building a
combined kernel for both V6 and V7. The code also needs to depend on ARMv7
with virtualization extensions, but that is a different issue. We don't
actually have a configuration symbol for that yet, as far as I can tell.

Arnd

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Re: [PATCH 5/9] ARM: Xen: fix initial build problems: [ In reply to ]

dan.magenheimer at oracle

Oct 9, 2012, 12:24 PM

Post #9 of 17 (440 views)

Permalink

> From: Ian Campbell [mailto:Ian.Campbell@citrix.com]
> Sent: Tuesday, October 09, 2012 9:36 AM
> To: Arnd Bergmann
> Cc: Jeremy Fitzhardinge; xen-devel@lists.xensource.com; Stefano Stabellini; Konrad Rzeszutek Wilk;
> linux-kernel@vger.kernel.org; Russell King; linux-arm-kernel@lists.infradead.org
> Subject: Re: [Xen-devel] [PATCH 5/9] ARM: Xen: fix initial build problems:
>
> On Tue, 2012-10-09 at 16:22 +0100, Arnd Bergmann wrote:
> > * The XEN_BALLOON code requires the balloon infrastructure that is not
> > getting built on ARM.
>
> I've got patches to enable this, but not for 3.7 so this looks good for
> now.
>
> > * The tmem hypercall is not available on ARM

[reduced cc list to just us Xen chickens]

Any reason that the tmem hypercall is not available on ARM?
At a minimum, it should be available, but always return failure.
Then an ARM Linux kernel can be ready for the Xen implementation
when the Xen hypervisor supports it.

Ballooning also... why disable it rather than just have
the hypervisor fail all calls?

Wondering if it just has never been tried/tested, or if there
are known problems since everything should have been coded
quite portably. (I do recall some arch-specific code in
the hypervisor, now that I think of it, but it was pretty small.)

I haven't been plugged in to Xen/ARM, so I don't know the
target market, but I do know that some
Android distros have been using the in-kernel tmem version
(called zcache) even though it is still a Linux staging driver,
so there probably will be demand for Xen tmem too.

Dan

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Re: [PATCH 5/9] ARM: Xen: fix initial build problems: [ In reply to ]

Ian.Campbell at citrix

Oct 10, 2012, 12:53 AM

Post #10 of 17 (435 views)

Permalink

On Tue, 2012-10-09 at 20:24 +0100, Dan Magenheimer wrote:
> > From: Ian Campbell [mailto:Ian.Campbell@citrix.com]
> > Sent: Tuesday, October 09, 2012 9:36 AM
> > To: Arnd Bergmann
> > Cc: Jeremy Fitzhardinge; xen-devel@lists.xensource.com; Stefano Stabellini; Konrad Rzeszutek Wilk;
> > linux-kernel@vger.kernel.org; Russell King; linux-arm-kernel@lists.infradead.org
> > Subject: Re: [Xen-devel] [PATCH 5/9] ARM: Xen: fix initial build problems:
> >
> > On Tue, 2012-10-09 at 16:22 +0100, Arnd Bergmann wrote:
> > > * The XEN_BALLOON code requires the balloon infrastructure that is not
> > > getting built on ARM.
> >
> > I've got patches to enable this, but not for 3.7 so this looks good for
> > now.
> >
> > > * The tmem hypercall is not available on ARM
>
> [reduced cc list to just us Xen chickens]
>
> Any reason that the tmem hypercall is not available on ARM?
> At a minimum, it should be available, but always return failure.

The hypercall is available from a hypervisor point of view, although it
will return -Esomething since some of the required arch functions are
still stubs.

The issue Arnd is fixing is that the client code in the kernel doesn't
currently compile because we haven't added all the required per-arch
scaffolding yet. Or at least that is the case for ballooning, I haven't
tried building tmem myself but I assume Arnd wouldn't be sending the
patch if it built.

I posted a series to enable ballooning on ARM last week (see
<1349363496.866.49.camel@zakaz.uk.xensource.com>).

Ian.

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Re: [PATCH 5/9] ARM: Xen: fix initial build problems: [ In reply to ]

Ian.Campbell at citrix

Oct 10, 2012, 12:54 AM

Post #11 of 17 (441 views)

Permalink

On Tue, 2012-10-09 at 19:21 +0100, Arnd Bergmann wrote:
> On Tuesday 09 October 2012, Stefano Stabellini wrote:
> > > config XEN
> > > bool "Xen guest support on ARM (EXPERIMENTAL)"
> > > depends on EXPERIMENTAL && ARM && OF
> > > + depends on !CPU_V6
> > > help
> > > Say Y if you want to run Linux in a Virtual Machine on Xen on ARM.
> >
> > Considering that we rely on the virtualization extensions, this one can
> > be:
> >
> > depends on CPU_V7
> >
> > The rest looks fine. I can submit a second patch to change !CPU_V6 into
> > CPU_V7 later, if you prefer.
>
> CPU_V6 and CPU_V7 are not exclusive, I saw the problem when building a
> combined kernel for both V6 and V7. The code also needs to depend on ARMv7
> with virtualization extensions, but that is a different issue. We don't
> actually have a configuration symbol for that yet, as far as I can tell.

I don't think the guest kernels (including dom0) need the extensions to
run under Xen, they are only need by Xen itself. The guests should just
see a relatively normal v7 processor.

Stefano, does that sound right?

Ian.

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Re: [PATCH 5/9] ARM: Xen: fix initial build problems: [ In reply to ]

Ian.Campbell at citrix

Oct 10, 2012, 2:46 AM

Post #12 of 17 (437 views)

Permalink

On Tue, 2012-10-09 at 19:19 +0100, Arnd Bergmann wrote:
> On Tuesday 09 October 2012, Ian Campbell wrote:
> > On Tue, 2012-10-09 at 16:39 +0100, Arnd Bergmann wrote:
> > > On Tuesday 09 October 2012, Ian Campbell wrote:
> > > > > * The tmem hypercall is not available on ARM
> > > > >
> > > > > * ARMv6 does not support cmpxchg on 16-bit words that are used in the
> > > >
> > > > missing the end of this sentence?
> > >
> > > Right, I meant to say
> > >
> > > * ARMv6 does not support cmpxchg on 16-bit words that are used in the
> > > Xen grant table code, so we must ensure that Xen support is only built
> > > on ARMv7-only kernels not combined ARMv6/v7 kernels.
> > >
> > > This should be fixed differently in the future.
> >
> > Is this is a build time failure because gcc/gas/etc refuses to generate
> > these instructions if it is configured for v6?
> >
> > I ask because if it is only a runtime issue then we can reason that if
> > we are running Xen specific grant table code, then we must be running on
> > Xen and therefore must necessarily be running on a v7 (because Xen only
> > support v7+virt extensions) even if the kernel happens to be capable of
> > running on v6 too.
>
> The underlying reason of course is that ARMv6 doesn't have those
> instructions. The symptom you see is a link error because gcc emits
> a reference to the (intentionally missing) __bad_cmpxchg() function
> from

OK, then your fix is the best one for now.

> [...]
> The possible solutions I can see for this are:
>
> * change the grant table format to use 32 bits for the flags on ARM
> * change the code to always cmpxchg the entire 32 bit word including the flags.

I'd need to check the grant table semantics to see if this will be
possible.

> * implement your own cmpxchg wrapper that may be implemented using a spinlock
> rather than cmpxchg if ARMv6 is enabled.

Even if ARMv6 is enabled the grant table code will never be running on
one so so it might be ok to just have our own wrapper which
unconditionally uses the v7 instruction? That might upset gas though.

>
> Arnd

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Re: [PATCH 5/9] ARM: Xen: fix initial build problems: [ In reply to ]

arnd at arndb

Oct 10, 2012, 4:07 AM

Post #13 of 17 (442 views)

Permalink

On Wednesday 10 October 2012, Ian Campbell wrote:
> > * implement your own cmpxchg wrapper that may be implemented using a spinlock
> > rather than cmpxchg if ARMv6 is enabled.
>
> Even if ARMv6 is enabled the grant table code will never be running on
> one so so it might be ok to just have our own wrapper which
> unconditionally uses the v7 instruction? That might upset gas though.

Yes, that would be possible. You can tell gas to ignore the instruction
set in this case. If you do this, you can implement the update functions
more efficiently using direct ldrexh/strexh in assembler to avoid doing
two nested loops. I assume that you don't need the v1 grant table
code on ARM anyway, so the only code you need to look at is

while (!((flags = *pflags) & GTF_transfer_committed)) {
if (sync_cmpxchg(pflags, flags, 0) == flags)
return 0;
cpu_relax();
}

which should transform nicely into a few lines of inline assembly.

Arnd

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Re: [PATCH 5/9] ARM: Xen: fix initial build problems: [ In reply to ]

stefano.stabellini at eu

Oct 10, 2012, 7:47 AM

Post #14 of 17 (434 views)

Permalink

On Wed, 10 Oct 2012, Ian Campbell wrote:
> On Tue, 2012-10-09 at 19:21 +0100, Arnd Bergmann wrote:
> > On Tuesday 09 October 2012, Stefano Stabellini wrote:
> > > > config XEN
> > > > bool "Xen guest support on ARM (EXPERIMENTAL)"
> > > > depends on EXPERIMENTAL && ARM && OF
> > > > + depends on !CPU_V6
> > > > help
> > > > Say Y if you want to run Linux in a Virtual Machine on Xen on ARM.
> > >
> > > Considering that we rely on the virtualization extensions, this one can
> > > be:
> > >
> > > depends on CPU_V7
> > >
> > > The rest looks fine. I can submit a second patch to change !CPU_V6 into
> > > CPU_V7 later, if you prefer.
> >
> > CPU_V6 and CPU_V7 are not exclusive, I saw the problem when building a
> > combined kernel for both V6 and V7. The code also needs to depend on ARMv7
> > with virtualization extensions, but that is a different issue. We don't
> > actually have a configuration symbol for that yet, as far as I can tell.
>
> I don't think the guest kernels (including dom0) need the extensions to
> run under Xen, they are only need by Xen itself. The guests should just
> see a relatively normal v7 processor.
>
> Stefano, does that sound right?

Keep in mind that we are using HVC to issue hypercalls, and HVC has been
introduced with the virtualization extensions, if I am not mistaken.

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Re: [PATCH 5/9] ARM: Xen: fix initial build problems: [ In reply to ]

stefano.stabellini at eu

Oct 10, 2012, 7:51 AM

Post #15 of 17 (433 views)

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On Wed, 10 Oct 2012, Ian Campbell wrote:
> On Tue, 2012-10-09 at 20:24 +0100, Dan Magenheimer wrote:
> > > From: Ian Campbell [mailto:Ian.Campbell@citrix.com]
> > > Sent: Tuesday, October 09, 2012 9:36 AM
> > > To: Arnd Bergmann
> > > Cc: Jeremy Fitzhardinge; xen-devel@lists.xensource.com; Stefano Stabellini; Konrad Rzeszutek Wilk;
> > > linux-kernel@vger.kernel.org; Russell King; linux-arm-kernel@lists.infradead.org
> > > Subject: Re: [Xen-devel] [PATCH 5/9] ARM: Xen: fix initial build problems:
> > >
> > > On Tue, 2012-10-09 at 16:22 +0100, Arnd Bergmann wrote:
> > > > * The XEN_BALLOON code requires the balloon infrastructure that is not
> > > > getting built on ARM.
> > >
> > > I've got patches to enable this, but not for 3.7 so this looks good for
> > > now.
> > >
> > > > * The tmem hypercall is not available on ARM
> >
> > [reduced cc list to just us Xen chickens]
> >
> > Any reason that the tmem hypercall is not available on ARM?
> > At a minimum, it should be available, but always return failure.
>
> The hypercall is available from a hypervisor point of view, although it
> will return -Esomething since some of the required arch functions are
> still stubs.

We are probably just missing the implementation of HYPERVISOR_tmem_op.

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Re: [PATCH 5/9] ARM: Xen: fix initial build problems: [ In reply to ]

Ian.Campbell at citrix

Oct 10, 2012, 7:53 AM

Post #16 of 17 (433 views)

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On Wed, 2012-10-10 at 15:47 +0100, Stefano Stabellini wrote:
> On Wed, 10 Oct 2012, Ian Campbell wrote:
> > On Tue, 2012-10-09 at 19:21 +0100, Arnd Bergmann wrote:
> > > On Tuesday 09 October 2012, Stefano Stabellini wrote:
> > > > > config XEN
> > > > > bool "Xen guest support on ARM (EXPERIMENTAL)"
> > > > > depends on EXPERIMENTAL && ARM && OF
> > > > > + depends on !CPU_V6
> > > > > help
> > > > > Say Y if you want to run Linux in a Virtual Machine on Xen on ARM.
> > > >
> > > > Considering that we rely on the virtualization extensions, this one can
> > > > be:
> > > >
> > > > depends on CPU_V7
> > > >
> > > > The rest looks fine. I can submit a second patch to change !CPU_V6 into
> > > > CPU_V7 later, if you prefer.
> > >
> > > CPU_V6 and CPU_V7 are not exclusive, I saw the problem when building a
> > > combined kernel for both V6 and V7. The code also needs to depend on ARMv7
> > > with virtualization extensions, but that is a different issue. We don't
> > > actually have a configuration symbol for that yet, as far as I can tell.
> >
> > I don't think the guest kernels (including dom0) need the extensions to
> > run under Xen, they are only need by Xen itself. The guests should just
> > see a relatively normal v7 processor.
> >
> > Stefano, does that sound right?
>
> Keep in mind that we are using HVC to issue hypercalls, and HVC has been
> introduced with the virtualization extensions, if I am not mistaken.

I think we can ignore that in this context since it is only used by bits
which are already virtualisation aware -- i.e. you wouldn't want to add
it to the Kconfig as a dependency for Xen.

Ian.

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Re: [PATCH 5/9] ARM: Xen: fix initial build problems: [ In reply to ]

stefano.stabellini at eu

Oct 10, 2012, 9:01 AM

Post #17 of 17 (434 views)

Permalink

On Wed, 10 Oct 2012, Ian Campbell wrote:
> On Wed, 2012-10-10 at 15:47 +0100, Stefano Stabellini wrote:
> > On Wed, 10 Oct 2012, Ian Campbell wrote:
> > > On Tue, 2012-10-09 at 19:21 +0100, Arnd Bergmann wrote:
> > > > On Tuesday 09 October 2012, Stefano Stabellini wrote:
> > > > > > config XEN
> > > > > > bool "Xen guest support on ARM (EXPERIMENTAL)"
> > > > > > depends on EXPERIMENTAL && ARM && OF
> > > > > > + depends on !CPU_V6
> > > > > > help
> > > > > > Say Y if you want to run Linux in a Virtual Machine on Xen on ARM.
> > > > >
> > > > > Considering that we rely on the virtualization extensions, this one can
> > > > > be:
> > > > >
> > > > > depends on CPU_V7
> > > > >
> > > > > The rest looks fine. I can submit a second patch to change !CPU_V6 into
> > > > > CPU_V7 later, if you prefer.
> > > >
> > > > CPU_V6 and CPU_V7 are not exclusive, I saw the problem when building a
> > > > combined kernel for both V6 and V7. The code also needs to depend on ARMv7
> > > > with virtualization extensions, but that is a different issue. We don't
> > > > actually have a configuration symbol for that yet, as far as I can tell.
> > >
> > > I don't think the guest kernels (including dom0) need the extensions to
> > > run under Xen, they are only need by Xen itself. The guests should just
> > > see a relatively normal v7 processor.
> > >
> > > Stefano, does that sound right?
> >
> > Keep in mind that we are using HVC to issue hypercalls, and HVC has been
> > introduced with the virtualization extensions, if I am not mistaken.
>
> I think we can ignore that in this context since it is only used by bits
> which are already virtualisation aware -- i.e. you wouldn't want to add
> it to the Kconfig as a dependency for Xen.

Considering that ARM CPU_V* symbols are not mutually exclusive, if Linux
had a CPU_V7_VIRTEXT symbol I don't see why we shouldn't add it to our
list of dependencies. It is more accurate than CPU_V7 after all.

But I don't think that it is necessary to introduce one now only for
Xen, because like you said, we only need HVC and that is only used in
virtualization aware code.

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