Re: [PATCH v3 0/4] implement vcpu preempted check

[Pan Xinhui]

å 2016/9/30 17:08, Paolo Bonzini åé:
> On 30/09/2016 10:52, Pan Xinhui wrote:
> > > x86 has no hypervisor support, and I'd like to understand the desired
> > > semantics first, so I don't think it should block this series.  In
> >
> > Once a guest do a hypercall or something similar, IOW, there is a
> > kvm_guest_exit. we think this is a lock holder preemption.
> > Adn PPC implement it in this way.
>
> Ok, good.
>
> > > particular, there are at least the following choices:
> > >
> > > 1) exit to userspace (5-10.000 clock cycles best case) counts as
> > > lock holder preemption
> > >
> > > 2) any time the vCPU thread not running counts as lock holder
> > > preemption
> > >
> > > To implement the latter you'd need a hypercall or MSR (at least as
> > > a slow path), because the KVM preempt notifier is only active
> > > during the KVM_RUN ioctl.
> >
> > seems a little expensive. :(
> > How many clock cycles it might cost.
>
> An MSR read is about 1500 clock cycles, but it need not be the fast path
> (e.g. use a bit to check if the CPU is running, if not use the MSR to
> check if the CPU is in userspace but the CPU thread is scheduled).  But
> it's not necessary if you are just matching PPC semantics.
>
> Then the simplest thing is to use the kvm_steal_time struct, and add a
> new field to it that replaces pad[0].  You can write a 0 to the flag in
> record_steal_time (not preempted) and a 1 in kvm_arch_vcpu_put
> (preempted).  record_steal_time is called before the VM starts running,
> immediately after KVM_RUN and also after every sched_in.
>
> If KVM doesn't implement the flag, it won't touch that field at all.  So
> the kernel can write a 0, meaning "not preempted", and not care if the
> hypervisor implements the flag or not: the answer will always be safe.
>
> The pointer to the flag can be placed in a per-cpu u32*, and again if
> the u32* is NULL that means "not preempted".
really nice suggestion!  That's what I want :)

thanks
xinhui

> Paolo
>
>
> > I am still looking for one shared struct between kvm and guest kernel on
> > x86.
> > and every time kvm_guest_exit/enter called, we store some info in it. So
> > guest kernel can check one vcpu is running or not quickly.
> >
> > thanks
> > xinhui
> >
> > > Paolo