Mailing List Archive

commit bfb69205376d94ff91b09a337c47fb665ee12da3
Author: Jan Beulich <jbeulich@suse.com>
AuthorDate: Wed Mar 27 12:29:33 2024 +0100
Commit: Jan Beulich <jbeulich@suse.com>
CommitDate: Wed Mar 27 12:29:33 2024 +0100

x86/PoD: tie together P2M update and increment of entry count

When not holding the PoD lock across the entire region covering P2M
update and stats update, the entry count - if to be incorrect at all -
should indicate too large a value in preference to a too small one, to
avoid functions bailing early when they find the count is zero. However,
instead of moving the increment ahead (and adjust back upon failure),
extend the PoD-locked region.

Fixes: 99af3cd40b6e ("x86/mm: Rework locking in the PoD layer")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: George Dunlap <george.dunlap@cloud.com>
master commit: cc950c49ae6a6690f7fc3041a1f43122c250d250
master date: 2024-03-21 09:48:10 +0100
---
xen/arch/x86/mm/p2m-pod.c | 15 ++++++++++++---
1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/xen/arch/x86/mm/p2m-pod.c b/xen/arch/x86/mm/p2m-pod.c
index 99dbcb3101..e903db9d93 100644
--- a/xen/arch/x86/mm/p2m-pod.c
+++ b/xen/arch/x86/mm/p2m-pod.c
@@ -1370,19 +1370,28 @@ mark_populate_on_demand(struct domain *d, unsigned long gfn_l,
}
}

+ /*
+ * P2M update and stats increment need to collectively be under PoD lock,
+ * to prevent code elsewhere observing PoD entry count being zero despite
+ * there actually still being PoD entries (created by the p2m_set_entry()
+ * invocation below).
+ */
+ pod_lock(p2m);
+
/* Now, actually do the two-way mapping */
rc = p2m_set_entry(p2m, gfn, INVALID_MFN, order,
p2m_populate_on_demand, p2m->default_access);
if ( rc == 0 )
{
- pod_lock(p2m);
p2m->pod.entry_count += 1UL << order;
p2m->pod.entry_count -= pod_count;
BUG_ON(p2m->pod.entry_count < 0);
- pod_unlock(p2m);
+ }
+
+ pod_unlock(p2m);

+ if ( rc == 0 )
ioreq_request_mapcache_invalidate(d);
- }
else if ( order )
{
/*
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.17