linux-kernel - Re: [PATCH 09/67] KVM: SVM: Track per-vCPU IRTEs using kvm_kernel

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Message-ID: <Z_knw1j6KeaEHVxr@google.com>
Date: Fri, 11 Apr 2025 07:32:47 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Sairaj Arun 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>
Subject: Re: [PATCH 09/67] KVM: SVM: Track per-vCPU IRTEs using
 kvm_kernel_irqfd structure

On Fri, Apr 11, 2025, Arun Kodilkar, Sairaj wrote:
> On 4/5/2025 1:08 AM, Sean Christopherson wrote:
> > diff --git a/include/linux/kvm_irqfd.h b/include/linux/kvm_irqfd.h
> > index 8ad43692e3bb..6510a48e62aa 100644
> > --- a/include/linux/kvm_irqfd.h
> > +++ b/include/linux/kvm_irqfd.h
> > @@ -59,6 +59,9 @@ struct kvm_kernel_irqfd {
> >   	struct work_struct shutdown;
> >   	struct irq_bypass_consumer consumer;
> >   	struct irq_bypass_producer *producer;
> > +
> > +	struct list_head vcpu_list;
> > +	void *irq_bypass_data;
> >   };
> >   #endif /* __LINUX_KVM_IRQFD_H */
> 
> Hi Sean,
> You missed to update the functions avic_set_pi_irte_mode and
> avic_update_iommu_vcpu_affinity, which iterate over the ir_list.

Well bugger, I did indeed.  And now I'm questioning my (hacky) testing, as I don't
see how avic_update_iommu_vcpu_affinity() survived.

Oh, wow.  This is disgustingly hilarious.  By dumb luck, the offset of the data
pointer relative to the list_head structure is the same in amd_svm_iommu_ir and
kvm_kernel_irqfd.  And the usage in avic_set_pi_irte_mode() and
avic_update_iommu_vcpu_affinity() only ever touches the data, not "svm".

So even though the structure is completely wrong, the math works out and
avic_set_pi_irte_mode() and avic_update_iommu_vcpu_affinity() unknowingly pass in
irq_bypass_data, and all is well.

struct amd_svm_iommu_ir {
	struct list_head node;	/* Used by SVM for per-vcpu ir_list */
	void *data;		/* Storing pointer to struct amd_ir_data */
	struct vcpu_svm *svm;
};


struct kvm_kernel_irqfd {
	...

	struct kvm_vcpu *irq_bypass_vcpu;
	struct list_head vcpu_list;
	void *irq_bypass_data;
};

Great catch!  And thanks for the reviews!