Messages in this thread | | | Date | Tue, 1 Mar 2022 17:47:52 +0700 | Subject | Re: [RFC PATCH 05/13] KVM: SVM: Update max number of vCPUs supported for x2AVIC mode | From | Suravee Suthikulpanit <> |
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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@amd.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
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