linux-kernel - Re: [PATCH 2/5 V5] Add functions to check if the host has stopped the vm

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Message-ID: <20111208152310.GB6301@mgebm.net>
Date:	Thu, 8 Dec 2011 10:23:10 -0500
From:	Eric B Munson <emunson@...bm.net>
To:	Avi Kivity <avi@...hat.com>
Cc:	mingo@...hat.com, hpa@...or.com, arnd@...db.de,
	ryanh@...ux.vnet.ibm.com, aliguori@...ibm.com, mtosatti@...hat.com,
	jeremy.fitzhardinge@...rix.com, levinsasha928@...il.com,
	Jan Kiszka <jan.kiszka@...mens.com>, kvm@...r.kernel.org,
	linux-arch@...r.kernel.org, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/5 V5] Add functions to check if the host has stopped
 the vm

On Wed, 07 Dec 2011, Avi Kivity wrote:

> On 12/05/2011 10:19 PM, Eric B Munson wrote:
> > When a host stops or suspends a VM it will set a flag to show this.  The
> > watchdog will use these functions to determine if a softlockup is real, or the
> > result of a suspended VM.
> >  
> > +bool kvm_check_and_clear_guest_paused(int cpu)
> > +{
> > +	bool ret = false;
> > +	struct pvclock_vcpu_time_info *src;
> > +
> > +	/*
> > +	 * per_cpu() is safe here because this function is only called from
> > +	 * timer functions where preemption is already disabled.
> > +	 */
> > +	WARN_ON(!in_atomic());
> > +	src = &per_cpu(hv_clock, cpu);
> 
> __get_cpu_var(); drop the cpu argument
> 

Will change for V6.

> > +	if ((src->flags & PVCLOCK_GUEST_STOPPED) != 0) {
> > +		src->flags = src->flags & (~PVCLOCK_GUEST_STOPPED);
> 
> Isn't this racy?  Between reading and writing src->flags, we can exit to
> the hypervisor and add/remove new flags.  The write then overrides those
> new flags.
> 

If I understand (please correct me if this is wrong) because this is only
called from the watchdog, which disables preemption, we should be protected
from something else writing to these flags.

> > +		ret = true;
> > +	}
> > +
> > +	return ret;
> > +}
> > +EXPORT_SYMBOL_GPL(kvm_check_and_clear_guest_paused);
> > +
> >  static struct clocksource kvm_clock = {
> >  	.name = "kvm-clock",
> >  	.read = kvm_clock_get_cycles,
> 
> 
> -- 
> error compiling committee.c: too many arguments to function
> 

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