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

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

[Suthikulpanit, Suravee]

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@amd.com>
---
 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));
-- 
1.8.3.1

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

[Graf, Alexander]

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@amd.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));
> 

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@amd.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));
>>
> 

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

[Suthikulpanit, Suravee]

Alex,

Actually, a second thought on this patch, I should have been using
AVIC_PHYSICAL_ID_ENTRY_HOST_PHYSICAL_ID_MASK --->

On 5/9/19 12:39 PM, Suthikulpanit, Suravee wrote:
> [CAUTION: External Email]
> 
> 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@amd.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))
                                            ^ HERE

It does the same thing, but would be easier to understand.

The AVIC_MAX_PHYSICAL_ID_COUNT is meant for representing the max number
of entry allowed for AVIC physical APIC ID table, which is a different thing.

I'll send out V2.

Suravee
>>>                return;
>>>
>>>        entry = READ_ONCE(*(svm->avic_physical_id_cache));
>>>
>>