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

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Date: Sun, 21 Jan 2024 19:17:26 +0800
From: Yi Wang <up2wing@...il.com>
To: 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
Cc: up2wing@...il.com
Subject: [v2 0/4] KVM: irqchip: synchronize srcu only if needed

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.

Changelog:
----------
v1->v2:
  - setup empty irq routing in kvm_create_vm
  - don't setup irq routing in x86 KVM_CAP_SPLIT_IRQCHIP
  - don't setup irq routing in s390 KVM_CREATE_IRQCHIP

v1: https://lore.kernel.org/kvm/20240112091128.3868059-1-foxywang@tencent.com/

Yi Wang (4):
  KVM: irqchip: add setup empty irq routing function
  KVM: setup empty irq routing when create vm
  KVM: x86: don't setup empty irq routing when KVM_CAP_SPLIT_IRQCHIP
  KVM: s390: don't setup dummy routing when KVM_CREATE_IRQCHIP

 arch/s390/kvm/kvm-s390.c | 13 +++++--------
 arch/x86/kvm/irq.h       |  1 -
 arch/x86/kvm/irq_comm.c  |  5 -----
 arch/x86/kvm/x86.c       |  3 ---
 include/linux/kvm_host.h |  1 +
 virt/kvm/irqchip.c       | 19 +++++++++++++++++++
 virt/kvm/kvm_main.c      |  4 ++++
 7 files changed, 29 insertions(+), 17 deletions(-)

-- 
2.39.3