Re: [RFC PATCH v2 1/5] KVM: SVM: Stop warning if x2AVIC feature bit alone is enabled

[Mario Limonciello <superm1@kernel.org>]

On 9/16/25 2:14 AM, Naveen N Rao wrote:
> On Mon, Sep 15, 2025 at 01:04:56PM -0700, Sean Christopherson wrote:
>> On Thu, Sep 04, 2025, Naveen N Rao (AMD) wrote:
>>> A platform can choose to disable AVIC by turning off the AVIC CPUID
>>> feature bit, while keeping x2AVIC CPUID feature bit enabled to indicate
>>> AVIC support for the x2APIC MSR interface. Since this is a valid
>>> configuration, stop printing a warning.
>>>
>>> Signed-off-by: Naveen N Rao (AMD) <naveen@kernel.org>
>>> ---
>>>   arch/x86/kvm/svm/avic.c | 8 +-------
>>>   1 file changed, 1 insertion(+), 7 deletions(-)
>>>
>>> diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
>>> index a34c5c3b164e..346cd23a43a9 100644
>>> --- a/arch/x86/kvm/svm/avic.c
>>> +++ b/arch/x86/kvm/svm/avic.c
>>> @@ -1101,14 +1101,8 @@ bool avic_hardware_setup(void)
>>>   	if (!npt_enabled)
>>>   		return false;
>>>   
>>> -	/* AVIC is a prerequisite for x2AVIC. */
>>> -	if (!boot_cpu_has(X86_FEATURE_AVIC) && !force_avic) {
>>> -		if (boot_cpu_has(X86_FEATURE_X2AVIC)) {
>>> -			pr_warn(FW_BUG "Cannot support x2AVIC due to AVIC is disabled");
>>> -			pr_warn(FW_BUG "Try enable AVIC using force_avic option");
>>
>> I agree with the existing code, KVM should treat this as a firmware bug, where
>> "firmware" could also be the host VMM.  AIUI, x2AVIC can't actualy work without
>> AVIC support, so enumerating x2AVIC without AVIC is pointless and unexpected.
> 
> There are platforms where this is the case though:
> 
> $ cpuid -1 -l 0x8000000A | grep -i avic
>        AVIC: AMD virtual interrupt controller  = false
>        X2AVIC: virtualized X2APIC              = true
>        extended LVT AVIC access changes        = true
> 
> The above is from Zen 4 (Phoenix), and my primary concern is that we
> will start printing a warning by default. Besides, there isn't much a
> user can do here (except start using force_avic, which will taint the
> kernel). Maybe we can warn only if AVIC is being explicitly enabled?
> 

I'd say if you need to say something downgrade it to info instead and 
not mark it as firmware bug.

> There is another aspect to this: if we are force-enabling AVIC, then
> this can serve as a way to discover support for x2AVIC mode (this is
> what we do currently).  Otherwise, we may want to force-enable x2AVIC
> based on cpu family/model.
> 
> 
> - Naveen
>