Subject: Re: [PATCH v12 01/84] KVM: arm64: Release pfn, i.e. put page, if copying MTE tags hits ZONE_DEVICE

From: Alex Bennée <[email-masked]>
Date: Wed, 31 Jul 2024 17:23:34 +0100

Sean Christopherson <[email-masked]> writes:

> Put the page reference acquired by gfn_to_pfn_prot() if
> kvm_vm_ioctl_mte_copy_tags() runs into ZONE_DEVICE memory.  KVM's less-
> than-stellar heuristics for dealing with pfn-mapped memory means that KVM
> can get a page reference to ZONE_DEVICE memory.
>
> Fixes: f0376edb1ddc ("KVM: arm64: Add ioctl to fetch/store tags in a guest")
> Signed-off-by: Sean Christopherson <[email-masked]>
> ---
>  arch/arm64/kvm/guest.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/arch/arm64/kvm/guest.c b/arch/arm64/kvm/guest.c
> index 11098eb7eb44..e1f0ff08836a 100644
> --- a/arch/arm64/kvm/guest.c
> +++ b/arch/arm64/kvm/guest.c
> @@ -1059,6 +1059,7 @@ int kvm_vm_ioctl_mte_copy_tags(struct kvm *kvm,
>  		page = pfn_to_online_page(pfn);
>  		if (!page) {
>  			/* Reject ZONE_DEVICE memory */
> +			kvm_release_pfn_clean(pfn);

I guess this gets renamed later in the series.

However my main comment is does lack of page always mean a ZONE_DEVICE?
Looking at pfn_to_online_page() I see a bunch of other checks first. Why
isn't it that functions responsibility to clean up after itself if its
returning NULLs?

>  			ret = -EFAULT;
>  			goto out;
>  		}

-- 
Alex Bennée
Virtualisation Tech Lead @ Linaro