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

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Message-ID: <CAAdeq_LmqKymD8J9tgEG5AXCXsJTQ1Z1XQan5nD-1qqUXv976w@mail.gmail.com>
Date: Tue, 12 Aug 2025 19:50:50 +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 Tue, Aug 12, David Woodhouse <dwmw2@...radead.org> wrote:
>
> On Tue, 2025-08-12 at 18:08 +0800, hugo lee wrote:
> >
> > On some legacy bios images using guests, they may disable PIT
> > after booting.
>
> Do you mean they may *not* disable the PIT after booting? Linux had
> that problem for a long time, until I fixed it with
> https://git.kernel.org/torvalds/c/70e6b7d9ae3
>

True, they disabled LINT0 and left PIT unaware.

> > When irqchip=split is on, qemu will keep kicking the guest and try to
> > get the Big QEMU Lock.
>
> If it's the PIT, surely QEMU will keep stealing time pointlessly unless
> we actually disable the PIT itself? Not just the IRQ delivery? Or do
> you use this to realise that the IRQ output from the PIT isn't going
> anywhere and thus disable the event in QEMU completely?
>

I'm using this to disable the PIT event in QEMU.

I'm aiming to solve the desynchronization caused by
irqchip=split, so the VM will behave more like the
physical one.
And this synchronization could eliminate the most
performance loss here.
The meaningless PIT which causes the pointless time
cost is guests' problem, and I don't
think we should disable it without clear instructions.