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

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Message-ID: <6a1d61925118b93badc777c2b9282d190c55a32b.camel@infradead.org>
Date: Tue, 12 Aug 2025 11:39:31 +0200
From: David Woodhouse <dwmw2@...radead.org>
To: Sean Christopherson <seanjc@...gle.com>, hugo lee <cs.hugolee@...il.com>
Cc: pbonzini@...hat.com, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
  dave.hansen@...ux.intel.com, hpa@...or.com, x86@...nel.org,
 kvm@...r.kernel.org,  linux-kernel@...r.kernel.org, Yuguo Li
 <hugoolli@...cent.com>
Subject: Re: [PATCH] KVM: x86: Synchronize APIC State with QEMU when
 irqchip=split

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@...gle.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.

Is there a corresponding QEMU patch to use this new exit reason?

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