Re: [PATCH v2 08/12] KVM: SVM: Update AVIC settings when changing APIC mode

[Suravee Suthikulpanit]

Maxim, Sean

On 4/18/22 7:55 PM, Maxim Levitsky wrote:
> On Tue, 2022-04-12 at 06:58 -0500, Suravee Suthikulpanit wrote:
> > When APIC mode is updated (e.g. disabled, xAPIC, or x2APIC),
> > KVM needs to call kvm_vcpu_update_apicv() to update AVIC settings
> > accordingly.
> >
> > Signed-off-by: Suravee Suthikulpanit<suravee.suthikulpanit@xxxxxxx>
> > ---
> >   arch/x86/kvm/svm/avic.c | 15 +++++++++++++++
> >   arch/x86/kvm/svm/svm.c  |  1 +
> >   2 files changed, 16 insertions(+)
> >
> > diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
> > index 22ee1098e2a5..01392b8364f4 100644
> > --- a/arch/x86/kvm/svm/avic.c
> > +++ b/arch/x86/kvm/svm/avic.c
> > @@ -616,6 +616,21 @@ void avic_apicv_post_state_restore(struct kvm_vcpu *vcpu)
> >   	avic_handle_ldr_update(vcpu);
> >   }
> >   
> > +void avic_set_virtual_apic_mode(struct kvm_vcpu *vcpu)
> > +{
> > +	struct vcpu_svm *svm = to_svm(vcpu);
> > +
> > +	if (!lapic_in_kernel(vcpu) || (avic_mode == AVIC_MODE_NONE))
> > +		return;
> > +
> > +	if (kvm_get_apic_mode(vcpu) == LAPIC_MODE_INVALID) {
> > +		WARN_ONCE(true, "Invalid local APIC state (vcpu_id=%d)", vcpu->vcpu_id);
> > +		return;
> > +	}
> > +
> > +	kvm_vcpu_update_apicv(&svm->vcpu);
> I think it makes sense to call avic_refresh_apicv_exec_ctrl directly here.
>   
> I am not sure that kvm_vcpu_update_apicv will even call it
> because it has an optimization of doing nothing when inhibition status
> didn't change.
>   
>   
> Another semi-related note:
>   
> the current way the x2avic msrs are configured creates slight performance
> problem for nesting:
>   
> The problem is that when entering a nested guest, AVIC on the current vCPU
> is inhibited, but this is done only so that this vCPU*peers*  don't
> try to use AVIC to send IPIs to it, so there is no need to update vmcb01
> msr interception bitmap, and vmcb02 should have all these msrs intercepted always.
> Same with returning to host.
>
> It also should be checked that during nested entry, at least vmcb01 msr bitmap
> is updated - TL;DR - please check that x2avic works when there is a nested guest running.

In the kvm/queue branch, I found a regression on nested SVM guest, where L2 guest cannot
launch. The bad commit is:

commit a4cfff3f0f8c07f1f7873a82bdeb3995807dac8c (bisect)
Merge: 42dcbe7d8bac 8d5678a76689
Author: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Date:   Fri Apr 8 12:43:40 2022 -0400

    Merge branch 'kvm-older-features' into HEAD

    Merge branch for features that did not make it into 5.18:

    * New ioctls to get/set TSC frequency for a whole VM

    * Allow userspace to opt out of hypercall patching

    Nested virtualization improvements for AMD:

    * Support for "nested nested" optimizations (nested vVMLOAD/VMSAVE,
      nested vGIF)

    * Allow AVIC to co-exist with a nested guest running

    * Fixes for LBR virtualizations when a nested guest is running,
      and nested LBR virtualization support

    * PAUSE filtering for nested hypervisors

    Guest support:

    * Decoupling of vcpu_is_preempted from PV spinlocks

    Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>

I am still working on the bisect into the merge commits.

Regards,
Suravee