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

From: Oliver Upton <[email-masked]>
Date: Fri, 31 May 2024 12:18:11 -0700

On Fri, May 31, 2024 at 12:11:33PM -0700, Oliver Upton wrote:
> 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.

Oh, and the WARN_ON() in kvm_pgtable_stage2_test_clear_young() is bogus
now. Maybe demote it to:

  r = kvm_pgtable_walk(...);
  WARN_ON_ONCE(r && r != -EAGAIN);

-- 
Thanks,
Oliver