[v6,11/11] KVM: xen: allow vcpu_info content to be 'safely' copied

Commit Message

Paul Durrant Sept. 26, 2023, 12:20 p.m. UTC

From: Paul Durrant <[e-mail]>

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.
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().

Signed-off-by: Paul Durrant <[e-mail]>
---
Cc: David Woodhouse <[e-mail]>
Cc: Sean Christopherson <[e-mail]>
Cc: Paolo Bonzini <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Borislav Petkov <[e-mail]>
Cc: Dave Hansen <[e-mail]>
Cc: "H. Peter Anvin" <[e-mail]>
Cc: [e-mail]

v6:
 - New in this version.
---
 arch/x86/kvm/xen.c | 3 ---
 1 file changed, 3 deletions(-)

Comments

David Woodhouse Sept. 26, 2023, 12:32 p.m. UTC | #1

On Tue, 2023-09-26 at 12:20 +0000, Paul Durrant wrote:
> From: Paul Durrant <[e-mail]>
> 
> 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.
> 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().

Reviewed-by: David Woodhouse <[e-mail]>

Patch

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