linux-kernel - Re: [KVM timekeeping 03/35] Move TSC offset writes to common code

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Date:	Mon, 23 Aug 2010 14:51:10 -1000
From:	Zachary Amsden <zamsden@...hat.com>
To:	Glauber Costa <glommer@...hat.com>
CC:	kvm@...r.kernel.org, Avi Kivity <avi@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	John Stultz <johnstul@...ibm.com>, linux-kernel@...r.kernel.org
Subject: Re: [KVM timekeeping 03/35] Move TSC offset writes to common code

On 08/20/2010 07:06 AM, Glauber Costa wrote:
> On Thu, Aug 19, 2010 at 10:07:17PM -1000, Zachary Amsden wrote:
>    
>> Also, ensure that the storing of the offset and the reading of the TSC
>> are never preempted by taking a spinlock.  While the lock is overkill
>> now, it is useful later in this patch series.
>>
>> +	spinlock_t tsc_write_lock;
>>      
> Forgive my utter ignorance, specially if it is to become
> obvious in a latter patch: This is a vcpu-local operation,
> uses rdtscl, so pcpu-local too, and we don't expect
> multiple writers to it at the same time.
>
> Why do we need this lock?
>
>    

Synchronizing access to the variables which we use to match TSC writes 
across multiple VCPUs.

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