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

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Date:   Thu, 9 May 2019 17:39:06 +0000
From:   "Suthikulpanit, Suravee" <Suravee.Suthikulpanit@....com>
To:     "Graf, Alexander" <graf@...zon.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

Alex,

On 5/7/19 9:16 AM, Graf, Alexander wrote:
> [CAUTION: External Email]
> 
> 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?

Actually, I would need to make change to that comment to remove the mentioning
of 255 as broadcast. I will send out V2 with proper comment fix.

The reason is because on system w/ x2APIC, the APIC ID 255 is actually
non-broadcast, and this should be allowed. The code here should not need
to check for broadcast.

Thanks,
Suravee

> 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));
>>
>