linux-kernel - Re: [PATCH v3 17/62] KVM: SVM: Add enable_ipiv param, never set IsRunning if disabled

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Message-ID: <aFVylP1XzMoqocOx@google.com>
Date: Fri, 20 Jun 2025 07:39:16 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Naveen N Rao <naveen@...nel.org>
Cc: Marc Zyngier <maz@...nel.org>, Oliver Upton <oliver.upton@...ux.dev>, 
	Paolo Bonzini <pbonzini@...hat.com>, Joerg Roedel <joro@...tes.org>, 
	David Woodhouse <dwmw2@...radead.org>, Lu Baolu <baolu.lu@...ux.intel.com>, 
	linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev, 
	kvm@...r.kernel.org, iommu@...ts.linux.dev, linux-kernel@...r.kernel.org, 
	Sairaj Kodilkar <sarunkod@....com>, Vasant Hegde <vasant.hegde@....com>, 
	Maxim Levitsky <mlevitsk@...hat.com>, Joao Martins <joao.m.martins@...cle.com>, 
	Francesco Lavra <francescolavra.fl@...il.com>, David Matlack <dmatlack@...gle.com>
Subject: Re: [PATCH v3 17/62] KVM: SVM: Add enable_ipiv param, never set
 IsRunning if disabled

On Thu, Jun 19, 2025, Naveen N Rao wrote:
> On Wed, Jun 11, 2025 at 03:45:20PM -0700, Sean Christopherson wrote:
> > From: Maxim Levitsky <mlevitsk@...hat.com>
> > 
> > Let userspace "disable" IPI virtualization for AVIC via the enable_ipiv
> > module param, by never setting IsRunning.  SVM doesn't provide a way to
> > disable IPI virtualization in hardware, but by ensuring CPUs never see
> > IsRunning=1, every IPI in the guest (except for self-IPIs) will generate a
> > VM-Exit.
> 
> I think this is good to have regardless of the erratum. Not sure about VMX,
> but does it make sense to intercept writes to the self-ipi MSR as well?

That doesn't work for AVIC, i.e. if the guest is MMIO to access the virtual APIC.

Regardless, I don't see any reason to manually intercept self-IPIs when IPI
virtualization is disabled.  AFAIK, there's no need to do so for correctness,
and Intel's self-IPI virtualization isn't tied to IPI virtualization either.
Self-IPI virtualization is enabled by virtual interrupt delivery, which in turn
is enabled by KVM when enable_apicv is true:

  Self-IPI virtualization occurs only if the “virtual-interrupt delivery”
  VM-execution control is 1.