Re: [PATCH v8 15/15] KVM: xen: allow vcpu_info content to be 'safely' copied

[Paul Durrant]

On 22/11/2023 10:39, David Woodhouse wrote:
> On Tue, 2023-11-21 at 22:53 +0000, David Woodhouse wrote:
> > On Tue, 2023-11-21 at 18:02 +0000, Paul Durrant wrote:
> > > From: Paul Durrant <pdurrant@xxxxxxxxxx>
> > >
> > > 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) there
> > > needs to be a way to defer that until the copy is complete.
> > > 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).
> > >
> > > 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, stop considering an inactive vcpu_info
> > > cache as a hard error in kvm_xen_set_evtchn_fast().
> > >
> > > Signed-off-by: Paul Durrant <pdurrant@xxxxxxxxxx>
> > > Reviewed-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
> >
> > Wait, didn't we realise that this leaves the bits set in the shadow
> > evtchn_pending_sel that get lost on migration?

Indeed we did not, but that's not something that *this* patch, or even 
this series, is dealing with.  We also know that setting the 'width' of 
shared_info has some issues, but again, can we keep that for other 
patches? The series is at v9 and has already suffered a fair amount of 
scope-creep.

  Paul