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

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Message-ID: <193779d766e08f4e013326da9f1de9de5e030398.camel@infradead.org>
Date: Thu, 14 Aug 2025 11:10:35 +0200
From: David Woodhouse <dwmw2@...radead.org>
To: hugo lee <cs.hugolee@...il.com>
Cc: Sean Christopherson <seanjc@...gle.com>, 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 Thu, 2025-08-14 at 16:54 +0800, hugo lee wrote:
> On Wed, Aug 13, 2025 David Woodhouse <dwmw2@...radead.org> wrote:
> > 
> > On Wed, 2025-08-13 at 17:30 +0800, hugo lee wrote:
> > > 
> > > Sorry for the misleading, what I was going to say is
> > > do only cpu_synchroniza_state() in this new userspace exit reason
> > > and do nothing on the PIT.
> > > So QEMU will ignore the PIT as the guests do.
> > > 
> > > The resample is great and needed, but the synchronization
> > > makes more sense to me on this question.
> > 
> > So if the guest doesn't actually quiesce the PIT, QEMU will *still*
> > keep waking up to waggle the PIT output pin, it's just that QEMU won't
> > bother telling the kernel about it?
> 
> Yes, just as guests wish.
> This could eliminate the most performance loss.
> 
> But I guess resample is more acceptable.

Simpler, cleaner, solves it for more use cases including when the
interrupt *is* delivered to the PIC but just never services. And allows
us to fix the VFIO INTx abomination and use the kernel's irqfd API
properly...

But that's a discussion for the qemu-devel list, I suppose.

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