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

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Message-ID: <CAAdeq_JQO2x=x07BwmDcTCU-WfKLW6kFk38881vbyUTehP+7gw@mail.gmail.com>
Date: Thu, 14 Aug 2025 16:54:48 +0800
From: hugo lee <cs.hugolee@...il.com>
To: David Woodhouse <dwmw2@...radead.org>
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 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.