Re: [RFC PATCH 05/13] KVM: SVM: Update max number of vCPUs supported for x2AVIC mode

[Suravee Suthikulpanit]

Hi Maxim,

On 2/25/22 12:18 AM, Maxim Levitsky wrote:
> On Sun, 2022-02-20 at 20:19 -0600, Suravee Suthikulpanit wrote:
> > xAVIC and x2AVIC modes can support diffferent number of vcpus.
> > Update existing logics to support each mode accordingly.
> >
> > Also, modify the maximum physical APIC ID for AVIC to 255 to reflect
> > the actual value supported by the architecture.
> >
> > Signed-off-by: Suravee Suthikulpanit<suravee.suthikulpanit@xxxxxxx>
> > ---
> >   arch/x86/include/asm/svm.h | 12 +++++++++---
> >   arch/x86/kvm/svm/avic.c    |  8 +++++---
> >   2 files changed, 14 insertions(+), 6 deletions(-)
> >
> > diff --git a/arch/x86/include/asm/svm.h b/arch/x86/include/asm/svm.h
> > index 7a7a2297165b..681a348a9365 100644
> > --- a/arch/x86/include/asm/svm.h
> > +++ b/arch/x86/include/asm/svm.h
> > @@ -250,10 +250,16 @@ enum avic_ipi_failure_cause {
> >   
> >   
> >   /*
> > - * 0xff is broadcast, so the max index allowed for physical APIC ID
> > - * table is 0xfe.  APIC IDs above 0xff are reserved.
> > + * For AVIC, the max index allowed for physical APIC ID
> > + * table is 0xff (255).
> >    */
> > -#define AVIC_MAX_PHYSICAL_ID_COUNT	0xff
> > +#define AVIC_MAX_PHYSICAL_ID		0XFFULL
> > +
> > +/*
> > + * For x2AVIC, the max index allowed for physical APIC ID
> > + * table is 0x1ff (511).
> > + */
> > +#define X2AVIC_MAX_PHYSICAL_ID		0x1FFUL
> Yep, physid page can't hold more entries...
>
> This brings the inventible question of what to do when a VM has more
> that 512 vCPUs...
>
> With AVIC, since it is xapic, it would be easy - xapic supports up to
> 254 CPUs.

Actually, 255 vCPUs.

> But with x2apic, there is no such restriction on max 512 CPUs,
> thus it is legal to create a VM with x2apic and more that 512 CPUs,
> and x2AVIC won't work well in this case.
>
> I guess AVIC_IPI_FAILURE_INVALID_TARGET, has to be extened to support those
> cases, even with loss of performance, or we need to inhibit x2AVIC.

In case of x2APIC-enabled guest w/ vCPU exceeding the max APIC ID (512) limit,
the ioctl operation for KVM_CREATE_VCPU will fail. For QEMU, this would
exit with error code. Would this be sufficient?

Regards,
Suravee