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Date:   Mon, 8 Oct 2018 16:12:23 +0800
From:   Yi Sun <yi.y.sun@linux.intel.com>
To:     Juergen Gross <jgross@suse.com>
Cc:     linux-kernel@vger.kernel.org, tglx@linutronix.de, mingo@kernel.org,
        sthemmin@microsoft.com, mikelley@microsoft.com, kys@microsoft.com,
        haiyangz@microsoft.com, hpa@zytor.com
Subject: Re: [tip:x86/hyperv] x86/hyperv: Enable PV qspinlock for Hyper-V
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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