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

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Message-ID: <c8950de7-bed3-06e0-9d4f-3d1ab1cad9a2@amd.com>
Date:   Thu, 9 May 2019 21:03:05 +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,

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@....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));
>>>
>>