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

[Sean Christopherson <seanjc@google.com>]

On Fri, Aug 08, 2025, hugo lee wrote:
> On Fri, Aug 8, 2025, Sean Christopherson <seanjc@google.com> 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.