linux-kernel - Re: [tip:x86/hyperv] x86/hyperv: Enable PV qspinlock for Hyper-V

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Message-ID: <alpine.DEB.2.21.1810021412060.12704@nanos.tec.linutronix.de>
Date:   Tue, 2 Oct 2018 14:13:14 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Juergen Gross <jgross@...e.com>
cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        sthemmin@...rosoft.com, mikelley@...rosoft.com, kys@...rosoft.com,
        haiyangz@...rosoft.com, yi.y.sun@...ux.intel.com, hpa@...or.com
Subject: Re: [tip:x86/hyperv] x86/hyperv: Enable PV qspinlock for Hyper-V

On Tue, 2 Oct 2018, Juergen Gross wrote:

> Sorry for noticing this only now, but I have been fighting with
> Xen PV qspinlocks last weekend:
> 
> > +	/*
> > +	 * Read HV_X64_MSR_GUEST_IDLE MSR can trigger the guest's
> > +	 * transition to the idle power state which can be exited
> > +	 * by an IPI even if IF flag is disabled.
> > +	 */
> 
> What if interrupts are enabled? Won't a kick happening here just
> interrupt and then the following rdmsr result in a hang?
> 
> I believe the correct way would be to:
> 
> - disable interrupts before above READ_ONCE() and restore them
>   after the rdmsrl()
> 
> - return early if in_nmi()
> 
> similar as the kvm specific variant is doing it.

Interesting question. I zapped the last commit for now until this is
clarified.

Thanks,

	tglx