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

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Message-ID: <97a694f4-322c-41ef-a3e7-c61fb867f6d6@amd.com>
Date: Fri, 11 Apr 2025 22:33:41 +0530
From: Sairaj Kodilkar <sarunkod@....com>
To: Sean Christopherson <seanjc@...gle.com>
CC: Paolo Bonzini <pbonzini@...hat.com>, Joerg Roedel <joro@...tes.org>,
	"David Woodhouse" <dwmw2@...radead.org>, Lu Baolu <baolu.lu@...ux.intel.com>,
	<kvm@...r.kernel.org>, <iommu@...ts.linux.dev>,
	<linux-kernel@...r.kernel.org>, Maxim Levitsky <mlevitsk@...hat.com>, "Joao
 Martins" <joao.m.martins@...cle.com>, David Matlack <dmatlack@...gle.com>,
	Naveen N Rao <naveen.rao@....com>, Vasant Hegde <vasant.hegde@....com>
Subject: Re: [PATCH 06/67] iommu/amd: WARN if KVM attempts to set vCPU
 affinity without posted intrrupts



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@...gle.com>
>>> ---
>>>    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)))
> 		return -EINVAL;
> 
> 	if (ir_data->iommu == NULL)
> 		return -EINVAL;
> 
> 	dev_data = search_dev_data(ir_data->iommu, irte_info->devid);
> 
> 	/* Note:
> 	 * This device has never been set up for guest mode.
> 	 * we should not modify the IRTE
> 	 */
> 	if (!dev_data || !dev_data->use_vapic)
> 		return -EINVAL;
> 
> I'd like to keep the WARN so that someone will notice if KVM screws up.

Yeah makes sense. Lets move it above

Thanks
Sairaj