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From: Yi Wang <up2wing@gmail.com>
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Cc: up2wing@gmail.com
Subject: [v4 0/3] KVM: irqchip: synchronize srcu only if needed
Date: Mon, 11 Mar 2024 19:31:43 +0800
Message-Id: <20240311113146.997631-1-foxywang@tencent.com>
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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 this patch has not been tested
thoroughly on s390 platform. Thanks to Christian for a quick test on
s390 and it still seems to work[1].

Changelog:
----------
v4:
  - replace loop with memset when setup empty irq routing table.

v3:
  - squash setup empty routing function and use of that into one commit
  - drop the comment in s390 part

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/

1. https://lore.kernel.org/lkml/f898e36f-ba02-4c52-a3be-06caac13323e@linux.ibm.com/


Yi Wang (3):
  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 |  9 +--------
 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, 25 insertions(+), 17 deletions(-)

-- 
2.39.3