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

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Date:   Thu, 24 Feb 2022 19:18:31 +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 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.

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.

Best regards,
	Maxim Levitsky

>  
>  #define AVIC_HPA_MASK	~((0xFFFULL << 52) | 0xFFF)
>  #define VMCB_AVIC_APIC_BAR_MASK		0xFFFFFFFFFF000ULL
> diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
> index 0040824e4376..1999076966fd 100644
> --- a/arch/x86/kvm/svm/avic.c
> +++ b/arch/x86/kvm/svm/avic.c
> @@ -195,7 +195,7 @@ void avic_init_vmcb(struct vcpu_svm *svm)
>  	vmcb->control.avic_backing_page = bpa & AVIC_HPA_MASK;
>  	vmcb->control.avic_logical_id = lpa & AVIC_HPA_MASK;
>  	vmcb->control.avic_physical_id = ppa & AVIC_HPA_MASK;
> -	vmcb->control.avic_physical_id |= AVIC_MAX_PHYSICAL_ID_COUNT;
> +	vmcb->control.avic_physical_id |= AVIC_MAX_PHYSICAL_ID;
>  	vmcb->control.avic_vapic_bar = APIC_DEFAULT_PHYS_BASE & VMCB_AVIC_APIC_BAR_MASK;
>  
>  	if (kvm_apicv_activated(svm->vcpu.kvm))
> @@ -210,7 +210,8 @@ static u64 *avic_get_physical_id_entry(struct kvm_vcpu *vcpu,
>  	u64 *avic_physical_id_table;
>  	struct kvm_svm *kvm_svm = to_kvm_svm(vcpu->kvm);
>  
> -	if (index >= AVIC_MAX_PHYSICAL_ID_COUNT)
> +	if ((avic_mode == AVIC_MODE_X1 && index > AVIC_MAX_PHYSICAL_ID) ||
> +	    (avic_mode == AVIC_MODE_X2 && index > X2AVIC_MAX_PHYSICAL_ID))
>  		return NULL;
>  
>  	avic_physical_id_table = page_address(kvm_svm->avic_physical_id_table_page);
> @@ -257,7 +258,8 @@ static int avic_init_backing_page(struct kvm_vcpu *vcpu)
>  	int id = vcpu->vcpu_id;
>  	struct vcpu_svm *svm = to_svm(vcpu);
>  
> -	if (id >= AVIC_MAX_PHYSICAL_ID_COUNT)
> +	if ((avic_mode == AVIC_MODE_X1 && id > AVIC_MAX_PHYSICAL_ID) ||
> +	    (avic_mode == AVIC_MODE_X2 && id > X2AVIC_MAX_PHYSICAL_ID))
>  		return -EINVAL;
>  
>  	if (!vcpu->arch.apic->regs)