Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO hypercall

Peter Zijlstra
2017-09-25T09:13:24Z

On Sun, Sep 24, 2017 at 11:57:53PM -0300, Marcelo Tosatti wrote:
> I think you are missing the following point:
>
> "vcpu0 can be interrupted when its not in a spinlock protected section,
> otherwise it can't."
>
> So you _have_ to communicate to the host when the guest enters/leaves a
> critical section.
>
> So this point of "everything needs to be RT and the priorities must be
> designed carefully", is this:
>
> WHEN in spinlock protected section (more specifically, when
> spinlock protected section _shared with realtime vcpus_),
>
> priority of vcpu0 > priority of emulator thread
>
> OTHERWISE
>
> priority of vcpu0 < priority of emulator thread.
>
> (*)
>
> So emulator thread can interrupt and inject interrupts to vcpu0.

spinlock protected regions are not everything. What about lock-free
constructs where CPU's spin-wait on one another (there's plenty).

And I'm clearly ignorant of how this emulation thread works, but why
would it run for a long time? Either it is needed for forward progress
of the VCPU or its not. If its not, it shouldn't run.