Mailing List Archive: sysenter/syscall support for 32-on-64 guests?

sysenter/syscall support for 32-on-64 guests?

Feb 28, 2008, 9:49 AM

Post #1 of 4 (58 views)

I just spent quite a while working out why pvops-Xen kernels weren't
booting for me in a 64-bit hypervisor. It turned out to be because
32-on-64 supports SEP even though 32-on-32 doesn't, so the guest kernel
was trying to use it without having set up the Xen side, resulting in
syscalls jumping to 0:0, generating a mysterious GP. Once I worked that
out, the fix was simple, of course.

Anyway, I wonder if you had a specific reason for allowing this, other
than "because we can"? It seems to me that we should try to minimize
the number of differences between 32-on-32 and 32-on-64 as much as possible.

J

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Re: sysenter/syscall support for 32-on-64 guests? [ In reply to ]

jbeulich at novell

Feb 28, 2008, 10:14 AM

Post #2 of 4 (55 views)

Permalink

>>> Jeremy Fitzhardinge <jeremy@goop.org> 28.02.08 17:49 >>>
>I just spent quite a while working out why pvops-Xen kernels weren't
>booting for me in a 64-bit hypervisor. It turned out to be because
>32-on-64 supports SEP even though 32-on-32 doesn't, so the guest kernel
>was trying to use it without having set up the Xen side, resulting in
>syscalls jumping to 0:0, generating a mysterious GP. Once I worked that
>out, the fix was simple, of course.
>
>Anyway, I wonder if you had a specific reason for allowing this, other
>than "because we can"? It seems to me that we should try to minimize
>the number of differences between 32-on-32 and 32-on-64 as much as possible.

Since int80 cannot be directly passed to the guest (as in 32-on-32), it
seemed reasonable to cut on the overhead of syscalls by at least
allowing this (and syscall could also be made work now that the vdso
stuff is unified in 2.6.25) - for obvious reasons it still goes through the
hypervisor, but gets there faster

Jan

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Re: sysenter/syscall support for 32-on-64 guests? [ In reply to ]

jeremy at goop

Feb 28, 2008, 1:19 PM

Post #3 of 4 (55 views)

Permalink

Jan Beulich wrote:
> Since int80 cannot be directly passed to the guest (as in 32-on-32), it
> seemed reasonable to cut on the overhead of syscalls by at least
> allowing this (and syscall could also be made work now that the vdso
> stuff is unified in 2.6.25) - for obvious reasons it still goes through the
> hypervisor, but gets there faster
>

I guess that's a reasonable point. But it does mean that you'd need to
separately validate a 32-bit guest on both a 32 and 64-bit hypervisor,
which is a bit painful. Maybe you'd need to do that anyway... (Or
ignore 32-bit hosts.)

J

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Re: Re: sysenter/syscall support for 32-on-64 guests? [ In reply to ]

keir.fraser at eu

Feb 28, 2008, 2:22 PM

Post #4 of 4 (54 views)

Permalink

On 28/2/08 17:14, "Jan Beulich" <jbeulich@novell.com> wrote:

>> Anyway, I wonder if you had a specific reason for allowing this, other
>> than "because we can"? It seems to me that we should try to minimize
>> the number of differences between 32-on-32 and 32-on-64 as much as possible.
>
> Since int80 cannot be directly passed to the guest (as in 32-on-32), it
> seemed reasonable to cut on the overhead of syscalls by at least
> allowing this (and syscall could also be made work now that the vdso
> stuff is unified in 2.6.25) - for obvious reasons it still goes through the
> hypervisor, but gets there faster

Ah yes, I must admit I temporarily forgot you can only target 64-bit code
segments in a long-mode IDT.

-- Keir

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