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

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Message-ID: <aFWws7h3L-iN52sF@google.com>
Date: Fri, 20 Jun 2025 12:04:19 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Oliver Upton <oliver.upton@...ux.dev>
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 Fri, Jun 20, 2025, Oliver Upton wrote:
> On Fri, Jun 20, 2025 at 10:22:32AM -0700, Sean Christopherson wrote:
> > On Fri, Jun 13, 2025, Oliver Upton wrote:
> > > On Thu, Jun 12, 2025 at 07:34:35AM -0700, Sean Christopherson wrote:
> > > > On Thu, Jun 12, 2025, Marc Zyngier wrote:
> > > > > 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.
> > 
> > Marc, does this look and read better?
> > 
> > I'd really, really like to get this sorted out asap, as it's the only thing
> > blocking the series, and I want to get the series into linux-next early next
> > week, before I go OOO for ~10 days.
> 
> Can you just send it out as a standalone patch? It's only tangientally
> related to the truckload of x86 stuff

The issue is that "KVM: Don't WARN if updating IRQ bypass route fails" directly
depends on both this patch and decent chunk of the x86 crud.  I could probably
trim some of the x86 crud by reshuffling patches around, but I can't get rid of
it entirely.

> that I'd rather not pull in the event of conflict resolution.

LOL, why not?  :-)

If I post it as a standalone patch, could you/Marc put it into a stable topic
branch based on kvm/master? (kvm/master now has patch 1, yay!)  Then I can create
a topic branch for this mountain of stuff based on the arm64 topic branch.