linux-kernel - Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT

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Date:   Mon, 25 Sep 2017 11:13:16 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Marcelo Tosatti <mtosatti@...hat.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        mingo@...hat.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO
 hypercall

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.