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

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Message-ID: <20111214175740.GC4075@mgebm.net>
Date:	Wed, 14 Dec 2011 12:57:40 -0500
From:	Eric B Munson <emunson@...bm.net>
To:	Avi Kivity <avi@...hat.com>
Cc:	Marcelo Tosatti <mtosatti@...hat.com>, mingo@...hat.com,
	hpa@...or.com, arnd@...db.de, ryanh@...ux.vnet.ibm.com,
	aliguori@...ibm.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, 14 Dec 2011, Avi Kivity wrote:

> On 12/14/2011 07:11 PM, Eric B Munson wrote:
> > > > > 
> > > > > 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.
> > > >
> > > > The host can write, but in that case race is harmless.
> > > 
> > > Why is it harmless?  You don't know what's in those other flags.
> > > 
> > > -- 
> > > error compiling committee.c: too many arguments to function
> > > 
> >
> > Currently there is only one other flag in this byte (PVCLOCK_TSC_STABLE_BIT)
> > and it isset once in kvmclock_init().  It is highly unlikely that the vm will
> > be stopped during this init and have the flag clobbered.  After the tsc stable
> > bit is written in the init, the field is read only outside of the guest paused
> > code.
> 
> Let's not lay minefields for us later to step into.  Making this a
> cpu-local atomic (irq safe but not smp safe) is simple and easy,
> somehting like __this_cpu_and().
> 
> -- 
> error compiling committee.c: too many arguments to function
> 

Fair enough, I will add this to V7.


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