linux-kernel - Re: Use of barriers in pvclock ABI

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Message-ID: <5d6222a80808110718i6a600858v7bdb5e08054ebefa@mail.gmail.com>
Date:	Mon, 11 Aug 2008 11:18:58 -0300
From:	"Glauber Costa" <glommer@...il.com>
To:	"Gerd Hoffmann" <kraxel@...hat.com>
Cc:	"Jeremy Fitzhardinge" <jeremy@...p.org>,
	"Avi Kivity" <avi@...ranet.com>,
	"Marcelo Tosatti" <mtosatti@...hat.com>,
	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
	kvm-devel <kvm-devel@...ts.sourceforge.net>
Subject: Re: Use of barriers in pvclock ABI

On Mon, Aug 11, 2008 at 4:08 AM, Gerd Hoffmann <kraxel@...hat.com> wrote:
>  Hi,
>
>> However, the pvclock_clocksource_read() implementation is
>> over-engineered, because it checks for an odd version and uses very
>> strong rmb() barriers (which generates either an "lfence" or "lock add
>> $0, (%esp)").
>>
>> If we're happy to guarantee as an ABI issue that the record will never
>> be updated cross-cpu, then we can make the barriers simply barrier() and
>> just check for (src->version != dst->version).
>>
>> Is that OK with you, or do you want to leave open the possibility of
>> doing cross-cpu time updates?
>
> Due to the TSC being involved here I don't expect cross-cpu time updates
> will ever happen.  IMHO it is fine to change that.

Okay for guest vcpu, but what about physical cpus?

IIRC, the checks are there, and so strict, to account for the
possiblity of the vcpu to be migrated to another cpu in the middle of
the
clock reading.
>
> cheers,
>  Gerd
>
> --
> http://kraxel.fedorapeople.org/xenner/
>



-- 
Glauber Costa.
"Free as in Freedom"
http://glommer.net

"The less confident you are, the more serious you have to act."
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