Re: [Xen-devel] Xen 4 TSC problems

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

On Thu, Sep 15, 2011 at 7:38 PM, Dan Magenheimer
<dan.magenheimer@xxxxxxxxxx> wrote:
>> I haven't been following this conversation, so I don't know if this is
>> relevant, but I've just discovered this morning that the TSC warp
>> check in Xen is done at the wrong time (before any secondary cpus are
>> brought up), and thus always returns warp=0.  I've submitted a patch
>> to do the check after secondary CPUs are brought up; that should cause
>> Xen to do periodic synchronization of TSCs when there is drift.
>
> Wow, nice catch, George!  I wonder if this is the underlying bug
> for many of the mysterious time problems that have been reported
> for a year or two now... at least on certain AMD boxes.
> Any idea when this was introduced?  Or has it always been wrong?

Well the comment in 20823:89907dab1aef seems to indicate that's where
the "assume it's reliable on AMD until proven otherwise" started; that
would be January 2010.

I looked as far back as 20705:a74aca4b9386, and there the TSC
reliability checks were again in init_xen_time().  Figuring out where
things were before then is getting into archeology. :-)

The comment at the top of init_xen_time() is correct now, but from the
time it was first written through 4.1 is was just plain wrong -- it
said init_xen_time() happened after all cpus were up, which has never
been true.

 -George

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