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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <aJobIRQ7Z4Ou1hz0@google.com>
Date: Mon, 11 Aug 2025 09:32:33 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: 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 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.