Re: [PATCH 6.1.y 2/2 V2] KVM: x86: Fix lapic timer interrupt lost after loading a snapshot.

[Sean Christopherson <seanjc@google.com>]

On Tue, Aug 27, 2024, Greg KH wrote:
> On Mon, Aug 26, 2024 at 06:13:36PM -0400, David Hunter wrote:
> > 
> > [ Upstream Commit 9cfec6d097c607e36199cf0cfbb8cf5acbd8e9b2]
> 
> This is already in the 6.1.66 release, so do you want it applied again?
> 
> > From: Haitao Shan <hshan@google.com>
> > Date:   Tue Sep 12 16:55:45 2023 -0700 
> > 
> > When running android emulator (which is based on QEMU 2.12) on
> > certain Intel hosts with kernel version 6.3-rc1 or above, guest
> > will freeze after loading a snapshot. This is almost 100%
> > reproducible. By default, the android emulator will use snapshot
> > to speed up the next launching of the same android guest. So
> > this breaks the android emulator badly.
> > 
> > I tested QEMU 8.0.4 from Debian 12 with an Ubuntu 22.04 guest by
> > running command "loadvm" after "savevm". The same issue is
> > observed. At the same time, none of our AMD platforms is impacted.
> > More experiments show that loading the KVM module with
> > "enable_apicv=false" can workaround it.
> > 
> > The issue started to show up after commit 8e6ed96cdd50 ("KVM: x86:
> > fire timer when it is migrated and expired, and in oneshot mode").
> > However, as is pointed out by Sean Christopherson, it is introduced
> > by commit 967235d32032 ("KVM: vmx: clear pending interrupts on
> > KVM_SET_LAPIC"). commit 8e6ed96cdd50 ("KVM: x86: fire timer when
> > it is migrated and expired, and in oneshot mode") just makes it
> > easier to hit the issue.
> > 
> > Having both commits, the oneshot lapic timer gets fired immediately
> > inside the KVM_SET_LAPIC call when loading the snapshot. On Intel
> > platforms with APIC virtualization and posted interrupt processing,
> > this eventually leads to setting the corresponding PIR bit. However,
> > the whole PIR bits get cleared later in the same KVM_SET_LAPIC call
> > by apicv_post_state_restore. This leads to timer interrupt lost.
> > 
> > The fix is to move vmx_apicv_post_state_restore to the beginning of
> > the KVM_SET_LAPIC call and rename to vmx_apicv_pre_state_restore.
> > What vmx_apicv_post_state_restore does is actually clearing any
> > former apicv state and this behavior is more suitable to carry out
> > in the beginning.
> > 
> > Fixes: 967235d32032 ("KVM: vmx: clear pending interrupts on KVM_SET_LAPIC")
> > Cc: stable@vger.kernel.org
> > Suggested-by: Sean Christopherson <seanjc@google.com>
> > Signed-off-by: Haitao Shan <hshan@google.com>
> > Link: https://lore.kernel.org/r/20230913000215.478387-1-hshan@google.com
> > Signed-off-by: Sean Christopherson <seanjc@google.com>
> > 
> > (Cherry-Picked from commit 9cfec6d097c607e36199cf0cfbb8cf5acbd8e9b2)
> > Signed-off-by: David Hunter <david.hunter.linux@gmail.com>
> > ---
> >  arch/x86/kvm/vmx/vmx.c | 1 +
> >  1 file changed, 1 insertion(+)
> > 
> > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > index 87abf4eebf8a..4040075bbd5a 100644
> > --- a/arch/x86/kvm/vmx/vmx.c
> > +++ b/arch/x86/kvm/vmx/vmx.c
> > @@ -8203,6 +8203,7 @@ static struct kvm_x86_ops vmx_x86_ops __initdata = {
> >  	.load_eoi_exitmap = vmx_load_eoi_exitmap,
> >  	.apicv_pre_state_restore = vmx_apicv_pre_state_restore,
> >  	.check_apicv_inhibit_reasons = vmx_check_apicv_inhibit_reasons,
> > +	.required_apicv_inhibits = VMX_REQUIRED_APICV_INHIBITS,
> >  	.hwapic_irr_update = vmx_hwapic_irr_update,
> >  	.hwapic_isr_update = vmx_hwapic_isr_update,
> >  	.guest_apic_has_interrupt = vmx_guest_apic_has_interrupt,
> 
> Wait, this is just one hunk?  This feels wrong, you didn't say why you
> modfied this from the original commit, or backport, what was wrong with
> that?

Gah, my bad.  I told David[*] that this needed to be paired with patch 1 to avoid
creating a regression in 6.1.y, without realizing this commit had already landed
in 6.1.y.

So yeah, please ignore this patch.

[*] https://lore.kernel.org/all/ZsSiQkQVSz0DarYC@google.com