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

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Message-ID: <basmpkilvhmg52pktjkpkqwmfcx2nnpbv75c6dxs73w7ehnwam@yyeft6vgabk7>
Date: Mon, 23 Jun 2025 16:15:13 +0530
From: Naveen N Rao <naveen@...nel.org>
To: Sean Christopherson <seanjc@...gle.com>
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 Fri, Jun 20, 2025 at 07:39:16AM -0700, Sean Christopherson wrote:
> 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.

Right, I was thinking about the Self-IPI MSR, but the ICR will also need 
to be intercepted.

> 
> 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.

Excellent, for this patch:
Reviewed-by: Naveen N Rao (AMD) <naveen@...nel.org>

Thanks,
Naveen