linux-kernel - Re: [RFCv2 PATCH 04/12] KVM: SVM: Update max number of vCPUs supported for x2AVIC mode

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Date:   Thu, 24 Mar 2022 13:13:57 +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: [RFCv2 PATCH 04/12] KVM: SVM: Update max number of vCPUs
 supported for x2AVIC mode

On Tue, 2022-03-08 at 10:39 -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
This should be 0xFE, since index 0xFF is reserved in AVIC mode.
It used to work because (see below) check used to be '>=',
but I do like that you switched to '>' check instead.


> +#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


>  
>  #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 49b185f0d42e..f128b0189d4a 100644
> --- a/arch/x86/kvm/svm/avic.c
> +++ b/arch/x86/kvm/svm/avic.c
> @@ -183,7 +183,7 @@ 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))
> @@ -198,7 +198,8 @@ 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)
This is the check I am talking about

> +	if ((avic_mode == AVIC_MODE_X1 && index > AVIC_MAX_PHYSICAL_ID) ||
> +	    (avic_mode == AVIC_MODE_X2 && index > X2AVIC_MAX_PHYSICAL_ID))
>  		return NULL;

I would probably like to ask to move this check to a function,
but I see that avic_get_physical_id_entry is only used in avic_handle_apic_id_update
in addition to avic_init_backing_page which has this check,
and I will sooner or later remove the anywat broken avic_handle_apic_id_update and
inline the avic_get_physical_id_entry probably so no need to do this.

>  
>  	avic_physical_id_table = page_address(kvm_svm->avic_physical_id_table_page);
> @@ -245,7 +246,8 @@ 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)


So except the off-by-one error in AVIC_MAX_PHYSICAL_ID_COUNT:

Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>

Best regards,
	Maxim Levitsky