Re: [RFC PATCH 12/13] KVM: SVM: Remove APICv inhibit reasone due to x2APIC

[Suravee Suthikulpanit]

Hi Maxim,

On 2/25/22 3:06 AM, Maxim Levitsky wrote:
> On Sun, 2022-02-20 at 20:19 -0600, Suravee Suthikulpanit wrote:
> > Currently, AVIC is inactive when booting a VM w/ x2APIC support.
> > With x2AVIC support, the APICV_INHIBIT_REASON_X2APIC can be removed.
> The commit title is a bit misleading - the inhibit reason is not removed,
> but rather AVIC is not inhibited when x2avic is present.

I'll fix the commit message.

> > Signed-off-by: Suravee Suthikulpanit<suravee.suthikulpanit@xxxxxxx>
> > ---
> >   arch/x86/kvm/svm/avic.c | 21 +++++++++++++++++++++
> >   arch/x86/kvm/svm/svm.c  | 18 ++----------------
> >   arch/x86/kvm/svm/svm.h  |  1 +
> >   3 files changed, 24 insertions(+), 16 deletions(-)
> >
> > diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
> > index 3306b74f1d8b..874c89f8fd47 100644
> > --- a/arch/x86/kvm/svm/avic.c
> > +++ b/arch/x86/kvm/svm/avic.c
> > @@ -21,6 +21,7 @@
> >   
> >   #include <asm/irq_remapping.h>
> >   
> > +#include "cpuid.h"
> >   #include "trace.h"
> >   #include "lapic.h"
> >   #include "x86.h"
> > @@ -176,6 +177,26 @@ void avic_vm_destroy(struct kvm *kvm)
> >   	spin_unlock_irqrestore(&svm_vm_data_hash_lock, flags);
> >   }
> >   
> > +void avic_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu, int nested)
> > +{
> > +	/*
> > +	 * If the X2APIC feature is exposed to the guest,
> > +	 * disable AVIC unless X2AVIC mode is enabled.
> > +	 */
> > +	if (avic_mode == AVIC_MODE_X1 &&
> > +	    guest_cpuid_has(vcpu, X86_FEATURE_X2APIC))
> > +		kvm_request_apicv_update(vcpu->kvm, false,
> > +					 APICV_INHIBIT_REASON_X2APIC);
> > +
> > +	/*
> > +	 * Currently, AVIC does not work with nested virtualization.
> > +	 * So, we disable AVIC when cpuid for SVM is set in the L1 guest.
> > +	 */
> > +	if (nested && guest_cpuid_has(vcpu, X86_FEATURE_SVM))
> > +		kvm_request_apicv_update(vcpu->kvm, false,
> > +					 APICV_INHIBIT_REASON_NESTED);
> BTW, now that I am thinking about it, it would be nice to be able to force
> the AVIC_MODE_X1 even if x2avic is present, for debug purposes from a module
> param. Just a suggestion.

Actually, we can force AVIC_MODE_X1 on a VM by disabling Guest x2APIC mode via QEMU option.
That should work for debugging purpose, right?

Regards,
Suravee