linux-kernel - Re: [PATCH v2] svm/avic: Allow avic_vcpu_load logic to support host APIC ID 255

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Message-ID: <035d681a-4ec6-05fd-a93b-a1bac2bfc0fe@redhat.com>
Date:   Mon, 20 May 2019 14:57:40 +0200
From:   Paolo Bonzini <pbonzini@...hat.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:     "rkrcmar@...hat.com" <rkrcmar@...hat.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "graf@...zon.com" <graf@...zon.com>
Subject: Re: [PATCH v2] svm/avic: Allow avic_vcpu_load logic to support host
 APIC ID 255

On 14/05/19 17:49, 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.
> 
> Also, use the more appropriate AVIC_PHYSICAL_ID_ENTRY_HOST_PHYSICAL_ID_MASK
> instead of AVIC_MAX_PHYSICAL_ID_COUNT. 
> 
> Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@....com>

Queued, with Cc to stable and using "kvm: svm/avic: fix off-by-one in
checking host APIC ID" as the subject.

Paolo

> ---
> 
> Change in V2:
>   * Use AVIC_PHYSICAL_ID_ENTRY_HOST_PHYSICAL_ID_MASK instead of
>     AVIC_MAX_PHYSICAL_ID_COUNT. 
> 
>  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 687767f..345fe9e 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_PHYSICAL_ID_ENTRY_HOST_PHYSICAL_ID_MASK))
>  		return;
>  
>  	entry = READ_ONCE(*(svm->avic_physical_id_cache));
>