From da662aecc830a9e56509b2d5bb99a9ccc683c24f Mon Sep 17 00:00:00 2001 From: Keir Fraser Date: Mon, 30 Jun 2025 09:39:33 +0000 Subject: [PATCH] FROMLIST: KVM: Avoid synchronize_srcu() in kvm_io_bus_register_dev() Device MMIO registration may happen quite frequently during VM boot, and the SRCU synchronization each time has a measurable effect on VM startup time. In our experiments it can account for around 25% of a VM's startup time. Replace the synchronization with a deferred free of the old kvm_io_bus structure. Bug: 395485007 Bug: 357781595 Link: https://lore.kernel.org/all/20250624092256.1105524-4-keirf@google.com/ Change-Id: I52af8db54259f4423e3dcdfa298573944796d88d Signed-off-by: Keir Fraser --- include/linux/kvm_host.h | 1 + virt/kvm/kvm_main.c | 10 ++++++++-- 2 files changed, 9 insertions(+), 2 deletions(-) diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h index 8f73d719e857..dd053429feb6 100644 --- a/include/linux/kvm_host.h +++ b/include/linux/kvm_host.h @@ -211,6 +211,7 @@ struct kvm_io_range { struct kvm_io_bus { int dev_count; int ioeventfd_count; + struct rcu_head rcu; struct kvm_io_range range[]; }; diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c index 889f740694a9..813259edb058 100644 --- a/virt/kvm/kvm_main.c +++ b/virt/kvm/kvm_main.c @@ -5985,6 +5985,13 @@ int kvm_io_bus_read(struct kvm_vcpu *vcpu, enum kvm_bus bus_idx, gpa_t addr, return r < 0 ? r : 0; } +static void __free_bus(struct rcu_head *rcu) +{ + struct kvm_io_bus *bus = container_of(rcu, struct kvm_io_bus, rcu); + + kfree(bus); +} + int kvm_io_bus_register_dev(struct kvm *kvm, enum kvm_bus bus_idx, gpa_t addr, int len, struct kvm_io_device *dev) { @@ -6023,8 +6030,7 @@ int kvm_io_bus_register_dev(struct kvm *kvm, enum kvm_bus bus_idx, gpa_t addr, memcpy(new_bus->range + i + 1, bus->range + i, (bus->dev_count - i) * sizeof(struct kvm_io_range)); rcu_assign_pointer(kvm->buses[bus_idx], new_bus); - synchronize_srcu_expedited(&kvm->srcu); - kfree(bus); + call_srcu(&kvm->srcu, &bus->rcu, __free_bus); return 0; }