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_ki0uZ9Rp3Fkrh1@google.com>
Date: Fri, 11 Apr 2025 07:10:26 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Sairaj Kodilkar <sarunkod@....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 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.