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

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Date:   Fri, 22 Sep 2017 12:56:09 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Marcelo Tosatti <mtosatti@...hat.com>
Cc:     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 Fri, Sep 22, 2017 at 12:00:04PM +0200, Peter Zijlstra wrote:
> On Thu, Sep 21, 2017 at 10:10:41PM -0300, Marcelo Tosatti wrote:
> > When executing guest vcpu-0 with FIFO:1 priority, which is necessary
> > to
> > deal with the following situation:
> > 
> > VCPU-0 (housekeeping VCPU)              VCPU-1 (realtime VCPU)
> > 
> > raw_spin_lock(A)
> > interrupted, schedule task T-1          raw_spin_lock(A) (spin)
> > 
> > raw_spin_unlock(A)
> > 
> > Certain operations must interrupt guest vcpu-0 (see trace below).
> 
> Those traces don't make any sense. All they include is kvm_exit and you
> can't tell anything from that.
> 
> > To fix this issue, only change guest vcpu-0 to FIFO priority
> > on spinlock critical sections (see patch).
> 
> This doesn't make sense. So you're saying that if you run all VCPUs as
> FIFO things come apart? Why?
> 
> And why can't they still come apart when the guest holds a spinlock?

That is, running a RT guest and not having _all_ VCPUs being RT tasks on
the host is absolutely and completely insane and broken.

Fix whatever needs fixing to allow your VCPU0 to be RT, don't do insane
things like this.