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

[Sairaj Kodilkar]

On 4/16/2025 3:34 AM, Sean Christopherson wrote:
> 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@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?
>
> 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.

Hi sean,
I think it is safe to have this WARN_ON(!dev_data->use_vapic) without
any false positives. IOMMU driver sets the dev_data->use_vapic only when
the device is in UNMANAGE_DOMAIN and it is 0 if the device is in any
other domain (DMA, DMA_FQ, IDENTITY).

We have a bigger problem from the VFIO side if we hit this WARN_ON()
as device is not in a UNMANGED_DOMAIN.

Regards
Sairaj Kodilkar