[PATCH 4/3] KVM: x86/xen: Make runstate cache gpcs share a lock

David Stevens
2023-01-30T05:26:17Z

From: David Stevens <stevensd@chromium.org>

Simplify locking in the case where the guest's runstate_info is split
across two pages by sharing a single lock for the two gpcs.

Signed-off-by: David Stevens <stevensd@chromium.org>
---
I tested this patch with xen_shinfo_test as suggested, and it still
passes. I agree that it makes sense to do this as a seperate patch. For
the bot reported issue, looks like I forgot to build with lockdep
enabled. I'll fix the issue with that patch in the next revision of the
series, so that there aren't any commits which don't build.

arch/x86/kvm/xen.c | 25 ++++---------------------
1 file changed, 4 insertions(+), 21 deletions(-)

diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
index fa8ab23271d3..9251f88a4e0d 100644
--- a/arch/x86/kvm/xen.c
+++ b/arch/x86/kvm/xen.c
@@ -310,24 +310,10 @@ static void kvm_xen_update_runstate_guest(struct kvm_vcpu *v, bool atomic)
update_bit = ((void *)(&rs_times[1])) - 1;
} else {
/*
- * The guest's runstate_info is split across two pages and we
- * need to hold and validate both GPCs simultaneously. We can
- * declare a lock ordering GPC1 > GPC2 because nothing else
- * takes them more than one at a time. Set a subclass on the
- * gpc1 lock to make lockdep shut up about it.
+ * The GPCs for both pages which comprise the guest's
+ * runstate_info share a lock, and it's already locked.
*/
- lock_set_subclass(gpc1->lock.dep_map, 1, _THIS_IP_);
- if (atomic) {
- if (!read_trylock(gpc2->lock)) {
- read_unlock_irqrestore(gpc1->lock, flags);
- return;
- }
- } else {
- read_lock(gpc2->lock);
- }
-
if (!kvm_gpc_check(gpc2, user_len2)) {
- read_unlock(gpc2->lock);
read_unlock_irqrestore(gpc1->lock, flags);

/* When invoked from kvm_sched_out() we cannot sleep */
@@ -427,9 +413,6 @@ static void kvm_xen_update_runstate_guest(struct kvm_vcpu *v, bool atomic)
smp_wmb();
}

- if (user_len2)
- read_unlock(gpc2->lock);
-
read_unlock_irqrestore(gpc1->lock, flags);

mark_page_dirty_in_slot(v->kvm, gpc1->memslot, gpc1->gpa >> PAGE_SHIFT);
@@ -2056,8 +2039,8 @@ void kvm_xen_init_vcpu(struct kvm_vcpu *vcpu)

kvm_gpc_init(&vcpu->arch.xen.runstate_cache, vcpu->kvm, NULL,
KVM_HOST_USES_PFN);
- kvm_gpc_init(&vcpu->arch.xen.runstate2_cache, vcpu->kvm, NULL,
- KVM_HOST_USES_PFN);
+ kvm_gpc_init_with_lock(&vcpu->arch.xen.runstate2_cache, vcpu->kvm, NULL,
+ KVM_HOST_USES_PFN, vcpu->arch.xen.runstate_cache.lock);
kvm_gpc_init(&vcpu->arch.xen.vcpu_info_cache, vcpu->kvm, NULL,
KVM_HOST_USES_PFN);
kvm_gpc_init(&vcpu->arch.xen.vcpu_time_info_cache, vcpu->kvm, NULL,
--
2.39.1.456.gfc5497dd1b-goog