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

[David Woodhouse]

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?

The point in your previous patch which split out a shiny new
set_shinfo_evtchn_pending() function was that you could then *call*
that function to ensure that the corresponding index bit was set on the
destination host after migration, if the bit in the shinfo is.

So we'd do that from kvm_xen_setup_evtchn(), kvm_xen_eventfd_assign(),
and when setting KVM_XEN_VCPU_ATTR_TYPE_TIMER.

 if (bit_is_set_in_shinfo)
   set_shinfo_evtchn_pending()

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