linux-kernel - Re: [PATCH v3 0/5] KVM: optimize userspace exits with a new ioctl

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Date:	Wed, 02 Sep 2015 12:31:37 +0200
From:	Christian Borntraeger <borntraeger@...ibm.com>
To:	Radim Krčmář <rkrcmar@...hat.com>,
	linux-kernel@...r.kernel.org
CC:	kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH v3 0/5] KVM: optimize userspace exits with a new ioctl

Am 14.08.2015 um 12:08 schrieb Radim Krčmář:
> v3:
>  * acked by Christian [1/5]
>  * use ioctl argument directly (unsigned long as flags) [4/5]
>  * precisely #ifdef arch-specific ioctls [5/5]
> v2:
>  * move request_exits debug counter patch right after introduction of
>    KVM_REQ_EXIT [3/5]
>  * use vcpu ioctl instead of vm one [4/5]
>  * shrink kvm_user_exit from 64 to 32 bytes [4/5]
>  * new [5/5]
> 
> QEMU uses SIGUSR1 to force a userspace exit and also to queue an early
> exit before calling VCPU_RUN -- the signal is blocked in user space and
> temporarily unblocked in VCPU_RUN.
> The temporal unblocking by sigprocmask() in kvm_arch_vcpu_ioctl_run()
> takes a shared siglock, which leads to cacheline bouncing in NUMA
> systems.
> 
> This series allows the same with a new request bit and VM IOCTL that
> marks and kicks target VCPU, hence no need to unblock.
> 
> inl_from_{pmtimer,qemu} vmexit benchmark from kvm-unit-tests shows ~5%
> speedup for 1-4 VCPUs (300-2000 saved cycles) without noticeably
> regressing kernel VM exits.
> (Paolo did a quick run of older version of this series on a NUMA system
>  and the speedup was around 35% when utilizing more nodes.)
> 
> Radim Krčmář (5):
>   KVM: add kvm_has_request wrapper
>   KVM: add KVM_REQ_EXIT request for userspace exit
>   KVM: x86: add request_exits debug counter
>   KVM: add KVM_USER_EXIT vcpu ioctl for userspace exit
>   KVM: refactor asynchronous vcpu ioctl dispatch
> 
>  Documentation/virtual/kvm/api.txt | 25 +++++++++++++++++++++++++
>  arch/x86/include/asm/kvm_host.h   |  1 +
>  arch/x86/kvm/vmx.c                |  4 ++--
>  arch/x86/kvm/x86.c                | 23 +++++++++++++++++++++++
>  include/linux/kvm_host.h          | 15 +++++++++++++--
>  include/uapi/linux/kvm.h          |  4 ++++
>  virt/kvm/kvm_main.c               | 15 ++++++++++-----
>  7 files changed, 78 insertions(+), 9 deletions(-)
> 

As far as I can see this should also work for s390 (when we implement 
REQ_EXIT handling)

To double check my understanding: these improvements come with a changed
userspace that does not use KVM_SET_SIGNAL_MASK and therefore this
conditional is false
	if (vcpu->sigset_active)
		sigprocmask(SIG_SETMASK, &vcpu->sigset, &sigsaved);
Correct?

That also means we improve exits that go to userspace but lightweight exits
that stay inside the kernel are not affected. 

Correct?

Christian

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