[v3 0/3] KVM: irqchip: synchronize srcu only if needed

From: Yi Wang
Date: Wed Jan 24 2024 - 06:35:15 EST


From: Yi Wang <foxywang@xxxxxxxxxxx>

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:
----------
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@xxxxxxxxxxx/

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(-)

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2.39.3