linux-kernel - Re: [v2 0/4] KVM: irqchip: synchronize srcu only if needed

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Message-ID: <CAN35MuQvQ7mbNCR=udA2xCu9wZ+qjSEM6eZ+6giJ8BBATsA-Ew@mail.gmail.com>
Date: Wed, 24 Jan 2024 20:44:56 +0800
From: Yi Wang <up2wing@...il.com>
To: Christian Borntraeger <borntraeger@...ux.ibm.com>
Cc: seanjc@...gle.com, pbonzini@...hat.com, tglx@...utronix.de, 
	mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com, x86@...nel.org, 
	hpa@...or.com, kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	wanpengli@...cent.com, foxywang@...cent.com, oliver.upton@...ux.dev, 
	maz@...nel.org, anup@...infault.org, atishp@...shpatra.org, 
	frankja@...ux.ibm.com, imbrenda@...ux.ibm.com
Subject: Re: [v2 0/4] KVM: irqchip: synchronize srcu only if needed

On Wed, Jan 24, 2024 at 8:14 PM Christian Borntraeger
<borntraeger@...ux.ibm.com> wrote:
>
> Am 21.01.24 um 12:17 schrieb Yi Wang:
> > From: Yi Wang <foxywang@...cent.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