Re: [PATCH 0/4] User return notifiers / just-in-time MSR switching for KVM

Avi Kivity
2009-10-25T12:23:23Z

On 09/16/2009 03:45 PM, Avi Kivity wrote:
> The various x86 syscall related MSRs (MSR_LSTAR and friends, EFER when SCE
> needs to be updated) are fairly expensive to read or write. Since different
> operating systems can set different values for these MSRs, KVM needs to reload
> them when switching to a different guest or to the host.
>
> Switching on every guest entry/exit is too expensive, so KVM reloads on
> guest preemption, which is a lot rarer. Even so, preemption based reload
> is suboptimal:
>
> - if we're switching to a kernel thread and back, there's no need to reload
> the MSRs. Examples of kernel threads we're likely to switch to are:
>
> - the idle task
> - a threaded interrupt handler
> - a kernel-mode virtio server (vhost-net)
>
> - if we're switching to a guest running the same OS, the MSRs will have the
> same values and there's no need to reload them
>
> - if the guest and host run the same OS, again the MSRs need not be reloaded.
>
> This patchset implements just-in-time reloads to defer them to the last
> possible instant. When we do reload, we check whether the values have in
> fact changed and reload conditionally.
>
> For the just-in-time reloads the first patch implements "user return
> notifiers", a callback invoked just before return to userspace. This has
> been written so that there is no code impact if KVM is not configured, and
> no runtime impact if KVM is not running.
>
> The patchset improves guest/idle/guest switches by about 2000 cycles.
>
>

I've applied this to kvm.git master.

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