Re: [PATCH] svm/avic: Allow avic_vcpu_load logic to support host APIC ID 255
From: Suthikulpanit, Suravee
Date: Thu May 09 2019 - 17:05:02 EST
Actually, a second thought on this patch, I should have been using
On 5/9/19 12:39 PM, Suthikulpanit, Suravee wrote:
> [CAUTION: External Email]
> 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.
>>> 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))
>>> - 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))
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.
>>> entry = READ_ONCE(*(svm->avic_physical_id_cache));