Re: [PATCH v12 02/84] KVM: arm64: Disallow copying MTE to guest memory while KVM is dirty logging

[Catalin Marinas <catalin.marinas@arm.com>]

On Fri, Jul 26, 2024 at 04:51:11PM -0700, Sean Christopherson wrote:
> Disallow copying MTE tags to guest memory while KVM is dirty logging, as
> writing guest memory without marking the gfn as dirty in the memslot could
> result in userspace failing to migrate the updated page.  Ideally (maybe?),
> KVM would simply mark the gfn as dirty, but there is no vCPU to work with,
> and presumably the only use case for copy MTE tags _to_ the guest is when
> restoring state on the target.
> 
> Fixes: f0376edb1ddc ("KVM: arm64: Add ioctl to fetch/store tags in a guest")
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>  arch/arm64/kvm/guest.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/arch/arm64/kvm/guest.c b/arch/arm64/kvm/guest.c
> index e1f0ff08836a..962f985977c2 100644
> --- a/arch/arm64/kvm/guest.c
> +++ b/arch/arm64/kvm/guest.c
> @@ -1045,6 +1045,11 @@ int kvm_vm_ioctl_mte_copy_tags(struct kvm *kvm,
>  
>  	mutex_lock(&kvm->slots_lock);
>  
> +	if (write && atomic_read(&kvm->nr_memslots_dirty_logging)) {
> +		ret = -EBUSY;
> +		goto out;
> +	}

There are ways to actually log the page dirtying but I don't think
it's worth it. AFAICT, reading the tags still works and that's what's
used during migration (on the VM where dirty tracking takes place).

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>