Re: [PATCH v7 11/11] KVM: xen: allow vcpu_info content to be 'safely' copied

[Sean Christopherson <seanjc@google.com>]

On Mon, Oct 02, 2023, Paul Durrant wrote:
> From: Paul Durrant <pdurrant@amazon.com>
> 
> If the guest sets an explicit vcpu_info GPA then, for any of the first 32
> vCPUs, the content of the default vcpu_info in the shared_info page must be
> copied into the new location. Because this copy may race with event
> delivery (which updates the 'evtchn_pending_sel' field in vcpu_info) we
> need a way to defer that until the copy is complete.

Nit, add a blank link between paragraphs.

> Happily there is already a shadow of 'evtchn_pending_sel' in kvm_vcpu_xen
> that is used in atomic context if the vcpu_info PFN cache has been
> invalidated so that the update of vcpu_info can be deferred until the
> cache can be refreshed (on vCPU thread's the way back into guest context).
> So let's also use this shadow if the vcpu_info cache has been
> *deactivated*, so that the VMM can safely copy the vcpu_info content and
> then re-activate the cache with the new GPA. To do this, all we need to do
> is stop considering an inactive vcpu_info cache as a hard error in
> kvm_xen_set_evtchn_fast().

Please, please try to write changelogs that adhere to the preferred style.  I
get that the preferred style likely doesn't align with what you're used to, but
the preferred style really doesn't help me get through reviews quicker.

> diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
> index aafc794940e4..e645066217bb 100644
> --- a/arch/x86/kvm/xen.c
> +++ b/arch/x86/kvm/xen.c
> @@ -1606,9 +1606,6 @@ int kvm_xen_set_evtchn_fast(struct kvm_xen_evtchn *xe, struct kvm *kvm)
>  		WRITE_ONCE(xe->vcpu_idx, vcpu->vcpu_idx);
>  	}
>  
> -	if (!vcpu->arch.xen.vcpu_info_cache.active)
> -		return -EINVAL;
> -

Hmm, maybe move this check after the "hard" error checks and explicitly do:

		return -EWOULDBLOCK

That way it's much more obvious that this patch is safe.  Alternatively, briefly
explain what happens if the cache is invalid in the changelog.


>  	if (xe->port >= max_evtchn_port(kvm))
>  		return -EINVAL;
>  
> -- 
> 2.39.2
>