From mboxrd@z Thu Jan  1 00:00:00 1970
From: Zachary Amsden <zamsden@redhat.com>
Subject: Re: [KVM timekeeping 03/35] Move TSC offset writes to common code
Date: Mon, 23 Aug 2010 14:51:10 -1000
Message-ID: <4C73177E.3050805@redhat.com>
References: <1282291669-25709-1-git-send-email-zamsden@redhat.com> <1282291669-25709-4-git-send-email-zamsden@redhat.com> <20100820170643.GB2937@mothafucka.localdomain>
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Cc: kvm@vger.kernel.org, Avi Kivity <avi@redhat.com>,
	Marcelo Tosatti <mtosatti@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	John Stultz <johnstul@us.ibm.com>, linux-kernel@vger.kernel.org
To: Glauber Costa <glommer@redhat.com>
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In-Reply-To: <20100820170643.GB2937@mothafucka.localdomain>
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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.