Re: [PATCH 06/67] iommu/amd: WARN if KVM attempts to set vCPU affinity without posted intrrupts

[Vasant Hegde]

On 4/11/2025 7:40 PM, Sean Christopherson wrote:
> On Fri, Apr 11, 2025, Sairaj Kodilkar wrote:
>> On 4/5/2025 1:08 AM, Sean Christopherson wrote:
>>> WARN if KVM attempts to set vCPU affinity when posted interrupts aren't
>>> enabled, as KVM shouldn't try to enable posting when they're unsupported,
>>> and the IOMMU driver darn well should only advertise posting support when
>>> AMD_IOMMU_GUEST_IR_VAPIC() is true.
>>>
>>> Note, KVM consumes is_guest_mode only on success.
>>>
>>> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
>>> ---
>>>   drivers/iommu/amd/iommu.c | 13 +++----------
>>>   1 file changed, 3 insertions(+), 10 deletions(-)
>>>
>>> diff --git a/drivers/iommu/amd/iommu.c b/drivers/iommu/amd/iommu.c
>>> index b3a01b7757ee..4f69a37cf143 100644
>>> --- a/drivers/iommu/amd/iommu.c
>>> +++ b/drivers/iommu/amd/iommu.c
>>> @@ -3852,19 +3852,12 @@ static int amd_ir_set_vcpu_affinity(struct irq_data *data, void *vcpu_info)
>>>   	if (!dev_data || !dev_data->use_vapic)
>>>   		return -EINVAL;
>>> +	if (WARN_ON_ONCE(!AMD_IOMMU_GUEST_IR_VAPIC(amd_iommu_guest_ir)))
>>> +		return -EINVAL;
>>> +
>>
>> Hi Sean,
>> 'dev_data->use_vapic' is always zero when AMD IOMMU uses legacy
>> interrupts i.e. when AMD_IOMMU_GUEST_IR_VAPIC(amd_iommu_guest_ir) is 0.
>> Hence you can remove this additional check.
> 
> Hmm, or move it above?  KVM should never call amd_ir_set_vcpu_affinity() if
> IRQ posting is unsupported, and that would make this consistent with the end
> behavior of amd_iommu_update_ga() and amd_iommu_{de,}activate_guest_mode().
> 
> 	if (WARN_ON_ONCE(!AMD_IOMMU_GUEST_IR_VAPIC(amd_iommu_guest_ir)))

Note that this is global IOMMU level check while dev_data->use_vapic is per
device. We set per device thing while attaching device to domain based on IOMMU
domain type and IOMMU vapic support.

How about add WARN_ON based on dev_data->use_vapic .. so that we can catch if
something went wrong in IOMMU side as well?

-Vasant