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

[Suthikulpanit, Suravee]

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@xxxxxxx>
> 
> 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
> 
>> ---
>> Â arch/x86/kvm/svm.c | 6 +++++-
>> Â 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)
>> ÂÂÂÂÂ if (!kvm_vcpu_apicv_active(vcpu))
>> ÂÂÂÂÂÂÂÂÂÂÂÂÂ return;
>>
>> -ÂÂÂÂ if (WARN_ON(h_physical_id >= AVIC_MAX_PHYSICAL_ID_COUNT))
>> +ÂÂÂÂ /*
>> +ÂÂÂÂÂ * Since the host physical APIC id is 8 bits,
>> +ÂÂÂÂÂ * we can support host APIC ID upto 255.
>> +ÂÂÂÂÂ */
>> +ÂÂÂÂ if (WARN_ON(h_physical_id > AVIC_MAX_PHYSICAL_ID_COUNT))
>> ÂÂÂÂÂÂÂÂÂÂÂÂÂ return;
>>
>> ÂÂÂÂÂ entry = READ_ONCE(*(svm->avic_physical_id_cache));
>>
>