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: <Z_7X3hoRdbHsTnc8@google.com>
Date: Tue, 15 Apr 2025 15:04:14 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Vasant Hegde <vasant.hegde@....com>
Cc: Sairaj Kodilkar <sarunkod@....com>, 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>
Subject: Re: [PATCH 06/67] iommu/amd: WARN if KVM attempts to set vCPU
 affinity without posted intrrupts

On Tue, Apr 15, 2025, Vasant Hegde wrote:
> 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)))
> 
> 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?

It's not clear to me that a WARN_ON(dev_data->use_vapic) would be free of false
positives.  AFAICT, the producers (e.g. VFIO) don't check whether or not a device
supports posting interrupts, and KVM definitely doesn't check.  And KVM is also
tolerant of irq_set_vcpu_affinity() failures, specifically for this type of
situation, so unfortunately I don't know that the IOMMU side of the world can
safely WARN.