linux-kernel - Re: [PATCH] svm/avic: Allow avic_vcpu_load logic to support host APIC ID 255

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Date:   Tue, 7 May 2019 16:16:19 +0200
From:   "Graf, Alexander" <graf@...zon.com>
To:     "Suthikulpanit, Suravee" <Suravee.Suthikulpanit@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
CC:     "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "rkrcmar@...hat.com" <rkrcmar@...hat.com>,
        "joro@...tes.org" <joro@...tes.org>
Subject: Re: [PATCH] svm/avic: Allow avic_vcpu_load logic to support host APIC
 ID 255

On 03.05.19 15:37, Suthikulpanit, Suravee wrote:
> Current logic does not allow VCPU to be loaded onto CPU with
> APIC ID 255. This should be allowed since the host physical APIC ID
> field in the AVIC Physical APIC table entry is an 8-bit value,
> and APIC ID 255 is valid in system with x2APIC enabled.
> 
> Instead, do not allow VCPU load if the host APIC ID cannot be
> represented by an 8-bit value.
> 
> Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@....com>

Your comment for AVIC_MAX_PHYSICAL_ID_COUNT says that 0xff (255) is 
broadcast hence you disallow that value. In fact, even the comment a few 
lines above the patch hunk does say that. Why the change of mind?

Alex

> ---
>   arch/x86/kvm/svm.c | 6 +++++-
>   1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index 294448e..122788f 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -2071,7 +2071,11 @@ static void avic_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>   	if (!kvm_vcpu_apicv_active(vcpu))
>   		return;
>   
> -	if (WARN_ON(h_physical_id >= AVIC_MAX_PHYSICAL_ID_COUNT))
> +	/*
> +	 * Since the host physical APIC id is 8 bits,
> +	 * we can support host APIC ID upto 255.
> +	 */
> +	if (WARN_ON(h_physical_id > AVIC_MAX_PHYSICAL_ID_COUNT))
>   		return;
>   
>   	entry = READ_ONCE(*(svm->avic_physical_id_cache));
>