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 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: ---------- v5: - rename new function to kvm_init_irq_routing as Sean suggested. - move invok of function of setting up empty before kvm_arch_init_vm. - Add necessary comment. 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@xxxxxxxxxxx/ 1. https://lore.kernel.org/lkml/f898e36f-ba02-4c52-a3be-06caac13323e@xxxxxxxxxxxxx/ 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 | 2 ++ virt/kvm/irqchip.c | 23 +++++++++++++++++++++++ virt/kvm/kvm_main.c | 9 ++++++++- 7 files changed, 34 insertions(+), 18 deletions(-) -- 2.39.3
