Re: [PATCH v4 6/7] KVM: arm64: Relax locking for kvm_test_age_gfn and kvm_age_gfn

On Wed, May 29, 2024 at 06:05:09PM +0000, James Houghton wrote:

[...]

diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c index 9e2bbee77491..eabb07c66a07 100644 --- a/arch/arm64/kvm/hyp/pgtable.c +++ b/arch/arm64/kvm/hyp/pgtable.c @@ -1319,10 +1319,8 @@ static int stage2_age_walker(const struct kvm_pgtable_visit_ctx *ctx, data->young = true; /*

• * stage2_age_walker() is always called while holding the MMU lock for
  

• * write, so this will always succeed. Nonetheless, this deliberately
  

• * follows the race detection pattern of the other stage-2 walkers in
  

• * case the locking mechanics of the MMU notifiers is ever changed.
  

• * This walk may not be exclusive; the PTE is permitted to change
  

• * from under us.
  

  */ if (data->mkold && !stage2_try_set_pte(ctx, new)) return -EAGAIN;

It is probably worth mentioning that if there was a race to update the PTE then the GFN is most likely young, so failing to clear AF probably isn't even consequential.