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

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Date:   Tue, 1 Mar 2022 17:47:52 +0700
From:   Suravee Suthikulpanit <suravee.suthikulpanit@....com>
To:     Maxim Levitsky <mlevitsk@...hat.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

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.

> 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