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

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Date:   Tue, 01 Mar 2022 13:31:32 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     pbonzini@...hat.com, seanjc@...gle.com, joro@...tes.org,
        jon.grimm@....com, wei.huang2@....com, terry.bowman@....com
Subject: Re: [RFC PATCH 05/13] KVM: SVM: Update max number of vCPUs
 supported for x2AVIC mode

On Tue, 2022-03-01 at 17:47 +0700, Suravee Suthikulpanit wrote:
> 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@....com>
> > > ---
> > >   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.

Sorry for off-by-one mistake - just remembered that 0xFF is reserved,
but then 255 is already 1 less that 256.

> 
> > 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?
Yes, this is the best.


Best regards,
	Maxim Levitsky


> 
> Regards,
> Suravee
> 
> 
>