On Thu, Sep 14, 2023, Binbin Wu wrote:
>
> On 9/14/2023 9:55 AM, Sean Christopherson wrote:
> > +void kvm_mmu_invalidate_end(struct kvm *kvm)
> > {
> > /*
> > * This sequence increase will notify the kvm page fault that
> > @@ -833,6 +848,13 @@ void kvm_mmu_invalidate_end(struct kvm *kvm, unsigned long start,
> > * in conjunction with the smp_rmb in mmu_invalidate_retry().
> > */
> > kvm->mmu_invalidate_in_progress--;
> > +
> > + /*
> > + * Assert that at least one range must be added between start() and
> > + * end(). Not adding a range isn't fatal, but it is a KVM bug.
> > + */
> > + WARN_ON_ONCE(kvm->mmu_invalidate_in_progress &&
> > + kvm->mmu_invalidate_range_start == INVALID_GPA);
> Should the check happen before the decrease of kvm->mmu_invalidate_in_progress?
> Otherwise, KVM calls kvm_mmu_invalidate_begin(), then kvm_mmu_invalidate_end()
> the check will not take effect.
Indeed. I'm pretty sure I added this code, not sure what I was thinking. There's
no reason to check mmu_invalidate_in_progress, it's not like KVM allows
mmu_invalidate_in_progress to go negative. The comment is also a bit funky. I'll
post a fixup patch to make it look like this (assuming I'm not forgetting a subtle
reason for guarding the check with the in-progress flag):
/*
* Assert that at least one range was added between start() and end().
* Not adding a range isn't fatal, but it is a KVM bug.
*/
WARN_ON_ONCE(kvm->mmu_invalidate_range_start == INVALID_GPA);
Regarding kvm->mmu_invalidate_in_progress, this would be a good opportunity to
move the BUG_ON() into the common end(), e.g. as is, an end() without a start()
from something other than the generic mmu_notifier would go unnoticed. And I
_think_ we can replace the BUG_ON() with a KVM_BUG_ON() without putting the
kernel at risk. E.g.
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index dd948276e5d6..54480655bcce 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -870,6 +870,7 @@ void kvm_mmu_invalidate_end(struct kvm *kvm)
* in conjunction with the smp_rmb in mmu_invalidate_retry().
*/
kvm->mmu_invalidate_in_progress--;
+ KVM_BUG_ON(kvm->mmu_invalidate_in_progress < 0, kvm);
/*
* Assert that at least one range was added between start() and end().
@@ -905,8 +906,6 @@ static void kvm_mmu_notifier_invalidate_range_end(struct mmu_notifier *mn,
*/
if (wake)
rcuwait_wake_up(&kvm->mn_memslots_update_rcuwait);
-
- BUG_ON(kvm->mmu_invalidate_in_progress < 0);
}
static int kvm_mmu_notifier_clear_flush_young(struct mmu_notifier *mn,
