Hi Yi, On 1/24/24 03:34, Yi Wang wrote: > 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. Would you mind explaining the reason that the *number of VMs* matters, as KVM_CAP_SPLIT_IRQCHIP is a per-VM cap? Or it meant it is more likely to have some VM workload impacted by the synchronize_srcu_expedited() as in prior discussion? https://lore.kernel.org/kvm/CAN35MuSkQf0XmBZ5ZXGhcpUCGD-kKoyTv9G7ya4QVD1xiqOxLg@xxxxxxxxxxxxxx/ Thank you very much! Dongli Zhang > > 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://urldefense.com/v3/__https://lore.kernel.org/kvm/20240112091128.3868059-1-foxywang@xxxxxxxxxxx/__;!!ACWV5N9M2RV99hQ!LjwKfBaGVl3u1l9YQSskg_1RU6278h2-fYnYLsoihF9i43aq73eIDqolGzOmeRvO8UlPreQHLqXEL1bAuw$ > > 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(-) >
