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

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Message-ID: <20181008081223.GG11769@yi.y.sun>
Date:   Mon, 8 Oct 2018 16:12:23 +0800
From:   Yi Sun <yi.y.sun@...ux.intel.com>
To:     Juergen Gross <jgross@...e.com>
Cc:     linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...nel.org,
        sthemmin@...rosoft.com, mikelley@...rosoft.com, kys@...rosoft.com,
        haiyangz@...rosoft.com, hpa@...or.com
Subject: Re: [tip:x86/hyperv] x86/hyperv: Enable PV qspinlock for Hyper-V

On 18-10-02 13:38:55, Juergen Gross wrote:
> > +static void hv_qlock_wait(u8 *byte, u8 val)
> > +{
> > +	unsigned long msr_val;
> > +
> > +	if (READ_ONCE(*byte) != val)
> > +		return;
> > +
> > +	/*
> > +	 * 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.
> 
> 
> Juergen

Thank you for the suggestion! That is a possible case. I will submit
a new version in soon.

BRs,
Yi Sun