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 12:11:33 -0700

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.

-- 
Thanks,
Oliver