linux-kernel - RE: [PATCH v2 4/4] KVM: Avoid synchronize_srcu() in kvm_io_bus_register

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [day] [month] [year] [list]

Message-ID: <b778c98abb4b425186bfeb1f9bed0c7a@baidu.com>
Date: Sat, 16 Aug 2025 00:23:30 +0000
From: "Li,Rongqing" <lirongqing@...du.com>
To: Keir Fraser <keirf@...gle.com>, "linux-arm-kernel@...ts.infradead.org"
	<linux-arm-kernel@...ts.infradead.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>
CC: Sean Christopherson <seanjc@...gle.com>, Eric Auger
	<eric.auger@...hat.com>, Oliver Upton <oliver.upton@...ux.dev>, Marc Zyngier
	<maz@...nel.org>, Will Deacon <will@...nel.org>, Paolo Bonzini
	<pbonzini@...hat.com>
Subject: RE: [PATCH v2 4/4] 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.
> 

Tested-by: Li RongQing <lirongqing@...du.com>

Thanks

-Li

> Signed-off-by: Keir Fraser <keirf@...gle.com>
> ---
>  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
> 9132148fb467..802ca46f7537 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -205,6 +205,7 @@ 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
> 9ec3b96b9666..f690a4997a0f 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -5948,6 +5948,13 @@ int kvm_io_bus_read(struct kvm_vcpu *vcpu,
> enum kvm_bus bus_idx, gpa_t addr,  }
> EXPORT_SYMBOL_GPL(kvm_io_bus_read);
> 
> +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)  { @@ -5986,8
> +5993,7 @@ 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;
>  }
> --
> 2.50.0.727.gbf7dc18ff4-goog
>