Re: [PATCH 07/22] KVM: arm64: nv: Set dirty state at stage-2

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] The kvm_s2_trans struct retains stale descriptor values and permissions because it is populated prior to handle_desc_update(), causing KVM to unnecessarily force a second VM exit for write faults on clean DBM pages.
--

commit c42a40795b5eef86805c06b6623a0ccb0c1d888d
Author: Oliver Upton <oupton@kernel.org>

KVM: arm64: nv: Set dirty state at stage-2
    
Set the dirty state on descriptors at stage-2 for write accesses when
DBM is set.

> diff --git a/arch/arm64/kvm/nested.c b/arch/arm64/kvm/nested.c
> index a70af3b3f05db..e5a407fc0880f 100644
> --- a/arch/arm64/kvm/nested.c
> +++ b/arch/arm64/kvm/nested.c
[ ... ]
> @@ -422,6 +430,20 @@ static int walk_nested_s2_pgd(struct kvm_vcpu *vcpu, struct kvm_walk_access *acc
>  	compute_s2_permissions(vcpu, wi, &ws, out);
>  	out->level = ws.level;
>  	out->desc = ws.desc;
> +
> +	ret = handle_desc_update(vcpu, wi, &ws, out, access);

[Severity: Medium]
Will this result in KVM mapping the shadow PTE read-only on the first fault,
causing a second VM exit?

Because compute_s2_permissions() calculates the out->dirty state based on the
pre-update descriptor, it seems out->dirty will remain false even if
handle_desc_update() successfully sets the dirty bit in guest memory.

When the walk returns, KVM's MMU consumes this in adjust_nested_fault_perms()
and strips KVM_PGTABLE_PROT_W, causing the L2 guest to retry the write and
immediately trigger a second VM exit.

Should out->dirty be updated to reflect the newly set dirty bit after
handle_desc_update() succeeds?

> +	if (ret)
> +		return ret;
[ ... ]

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260623184201.1518871-1-oupton@kernel.org?part=7