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Date: Wed, 24 Jan 2024 13:14:40 +0100
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Subject: Re: [v2 0/4] KVM: irqchip: synchronize srcu only if needed
To: Yi Wang <up2wing@gmail.com>, 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
References: <20240121111730.262429-1-foxywang@tencent.com>
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From: Christian Borntraeger <borntraeger@linux.ibm.com>
In-Reply-To: <20240121111730.262429-1-foxywang@tencent.com>
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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.