Re: [PATCH] KVM: x86: Synchronize APIC State with QEMU when irqchip=split

[hugo lee]

On Tue, Aug 12, 2025 at 5:39 PM David Woodhouse <dwmw2@xxxxxxxxxxxxx> wrote:
>
> On Mon, 2025-08-11 at 09:32 -0700, Sean Christopherson wrote:
> > On Fri, Aug 08, 2025, hugo lee wrote:
> > > On Fri, Aug 8, 2025, Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
> > > >
> > > > On Thu, Aug 07, 2025, hugo lee wrote:
> > > > > On Thu, Aug 7, 2025 Sean Christopherson wrote:
> > > > > >
> > > > > > On Wed, Aug 06, 2025, Yuguo Li wrote:
> > > > > > > When using split irqchip mode, IOAPIC is handled by QEMU while the LAPIC is
> > > > > > > emulated by KVM.  When guest disables LINT0, KVM doesn't exit to QEMU for
> > > > > > > synchronization, leaving IOAPIC unaware of this change.  This may cause vCPU
> > > > > > > to be kicked when external devices(e.g. PIT)keep sending interrupts.
> > > > > >
> > > > > > I don't entirely follow what the problem is.  Is the issue that QEMU injects an
> > > > > > IRQ that should have been blocked?  Or is QEMU forcing the vCPU to exit unnecessarily?
> > > > > >
> > > > >
> > > > > This issue is about QEMU keeps injecting should-be-blocked
> > > > > (blocked by guest and qemu just doesn't know that) IRQs.
> > > > > As a result, QEMU forces vCPU to exit unnecessarily.
> > > >
> > > > Is the problem that the guest receives spurious IRQs, or that QEMU is forcing
> > > > unnecesary exits, i.e hurting performance?
> > > >
> > >
> > > It is QEMU is forcing unnecessary exits which will hurt performance by
> > > trying to require the Big QEMU Lock in qemu_wait_io_event.
> >
> > Please elaborate on the performance impact and why the issue can't be solved in
> > QEMU.
>
> Is there a corresponding QEMU patch to use this new exit reason?

No, but the patch is done and will be submitted soon.