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

Sean Christopherson
2025-06-20T19:04:21Z

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.