linux-kernel - Re: [PATCH v3 02/62] KVM: arm64: WARN if unmapping vLPI fails

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Message-ID: <aEyOcJJsys9mm_Xs@linux.dev>
Date: Fri, 13 Jun 2025 13:47:44 -0700
From: Oliver Upton <oliver.upton@...ux.dev>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Marc Zyngier <maz@...nel.org>, 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 02/62] KVM: arm64: WARN if unmapping vLPI fails

On Thu, Jun 12, 2025 at 07:34:35AM -0700, Sean Christopherson wrote:
> On Thu, Jun 12, 2025, Marc Zyngier wrote:
> > On Wed, 11 Jun 2025 23:45:05 +0100,
> > Sean Christopherson <seanjc@...gle.com> wrote:
> > > 
> > > WARN if unmapping a vLPI in kvm_vgic_v4_unset_forwarding() fails, as
> > > failure means an IRTE has likely been left in a bad state, i.e. IRQs
> > > won't go where they should.
> > 
> > I have no idea what an IRTE is.
> 
> Sorry, x86 IOMMU terminology (Interrupt Remapping Table Entry).  I think the GIC
> terminology would be ITS entry?  Or maybe ITS mapping?

We tend to just call it a 'vLPI mapping', which under the hood implies
a couple other translations have been wired up as well (vPE + Device).

> > But not having an VLPI mapping for an interrupt at the point where we're
> > tearing down the forwarding is pretty benign. IRQs *still* go where they
> > should, and we don't lose anything.

The VM may not actually be getting torn down, though. The series of
fixes [*] we took for 6.16 addressed games that VMMs might be playing on
irqbypass for a live VM.

[*] https://lore.kernel.org/kvmarm/20250523194722.4066715-1-oliver.upton@linux.dev/

> All of those failure scenario seem like warnable offences when KVM thinks it has
> configured the IRQ to be forwarded to a vCPU.

I tend to agree here, especially considering how horribly fragile GICv4
has been in some systems. I know of a couple implementations where ITS
command failures and/or unmapped MSIs are fatal for the entire machine.
Debugging them has been a genuine pain in the ass.

WARN'ing when state tracking for vLPIs is out of whack would've made it
a little easier.

Thanks,
Oliver