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

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Date:   Mon, 2 Oct 2017 14:48:11 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Marcelo Tosatti <mtosatti@...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 Mon, Oct 02, 2017 at 02:30:33PM +0200, Paolo Bonzini wrote:
> I don't understand why is it correct to delay interrupt injection just
> because VCPU0 is running in a spinlock-protected region?  I just cannot
> see the reason why it's safe and not a recipe for priority inversions.

It is indeed not right. Something like:

	raw_spin_lock(&some_lock);

	/* do crud */

	raw_spin_unlock(&some_lock);

Should not hold off the interrupt that tells you your finger is in
imminent danger of becoming detached. Only when we do
local_irq_disable() (ie. raw_spin_lock_irq*() and the like) should we
avoid interrupt delivery.

This whole fixation on spinlock regions is misguided and must stop, its
wrong on all levels.