linux-kernel - Re: [v5 0/3] KVM: irqchip: synchronize srcu only if needed

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Date: Tue, 11 Jun 2024 18:18:29 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Sean Christopherson <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, 
	borntraeger@...ux.ibm.com, frankja@...ux.ibm.com, imbrenda@...ux.ibm.com, 
	weijiang.yang@...el.com, Yi Wang <up2wing@...il.com>
Subject: Re: [v5 0/3] KVM: irqchip: synchronize srcu only if needed

On Mon, 06 May 2024 18:17:48 +0800, Yi Wang wrote:
> 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.
> 
> [...]

Applied to kvm-x86 generic, with a lot of fixup.  A sanity check on the end
result would be much appreciated.  Thanks!

[1/3] KVM: Setup empty IRQ routing when creating a VM
      https://github.com/kvm-x86/linux/commit/fbe4a7e881d4
[2/3] KVM: x86: Don't re-setup empty IRQ routing when KVM_CAP_SPLIT_IRQCHIP
      https://github.com/kvm-x86/linux/commit/e3c89f5dd11d
[3/3] KVM: s390: Don't re-setup dummy routing when KVM_CREATE_IRQCHIP
      https://github.com/kvm-x86/linux/commit/c4201bd24f4a

--
https://github.com/kvm-x86/linux/tree/next