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

[Yi Wang]

On Wed, Jan 24, 2024 at 8:14 PM Christian Borntraeger
<borntraeger@xxxxxxxxxxxxx> wrote:
>
> Am 21.01.24 um 12:17 schrieb Yi Wang:
> > From: Yi Wang <foxywang@xxxxxxxxxxx>
> >
> > 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!

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Best wishes
Yi Wang