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From: Yi Wang <up2wing@gmail.com>
Date: Wed, 24 Jan 2024 20:44:56 +0800
Message-ID: <CAN35MuQvQ7mbNCR=udA2xCu9wZ+qjSEM6eZ+6giJ8BBATsA-Ew@mail.gmail.com>
Subject: Re: [v2 0/4] KVM: irqchip: synchronize srcu only if needed
To: Christian Borntraeger <borntraeger@linux.ibm.com>
Cc: seanjc@google.com, pbonzini@redhat.com, tglx@linutronix.de, 
	mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, x86@kernel.org, 
	hpa@zytor.com, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, 
	wanpengli@tencent.com, foxywang@tencent.com, oliver.upton@linux.dev, 
	maz@kernel.org, anup@brainfault.org, atishp@atishpatra.org, 
	frankja@linux.ibm.com, imbrenda@linux.ibm.com
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On Wed, Jan 24, 2024 at 8:14=E2=80=AFPM Christian Borntraeger
<borntraeger@linux.ibm.com> wrote:
>
> Am 21.01.24 um 12:17 schrieb Yi Wang:
> > From: Yi Wang <foxywang@tencent.com>
> >
> > We found that it may cost more than 20 milliseconds very accidentally
> > to enable cap of KVM_CAP_SPLIT_IRQCHIP on a host which has many vms
> > already.
> >
> > The reason is that when vmm(qemu/CloudHypervisor) invokes
> > KVM_CAP_SPLIT_IRQCHIP kvm will call synchronize_srcu_expedited() and
> > might_sleep and kworker of srcu may cost some delay during this period.
> > One way makes sence is setup empty irq routing when creating vm and
> > so that x86/s390 don't need to setup empty/dummy irq routing.
> >
> > Note: I have no s390 machine so the s390 patch has not been tested.
>
> I just did a quick sniff and it still seems to work. No performance check=
 etc.

Thanks very much, Christian!

---
Best wishes
Yi Wang