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Date: Tue, 11 Jun 2024 18:18:29 -0700
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Subject: Re: [v5 0/3] KVM: irqchip: synchronize srcu only if needed
From: Sean Christopherson <seanjc@google.com>
To: Sean Christopherson <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, 
	borntraeger@linux.ibm.com, frankja@linux.ibm.com, imbrenda@linux.ibm.com, 
	weijiang.yang@intel.com, Yi Wang <up2wing@gmail.com>
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On Mon, 06 May 2024 18:17:48 +0800, Yi Wang wrote:
> 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.
> 
> [...]

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