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RE: [Xen-devel] write_tsc in a PV domain?

To: Chris Lalancette <clalance@xxxxxxxxxx>
Subject: RE: [Xen-devel] write_tsc in a PV domain?
From: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
Date: Thu, 27 Aug 2009 06:00:58 -0700 (PDT)
Cc: Jeremy Fitzhardinge <jeremy@xxxxxxxx>, "Xen-Devel \(E-mail\)" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
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> Dan Magenheimer wrote:
> > In that case, are you saying it is an illegal instruction for a PV
> > guest to execute?  If so, we should not ignore it, we should fail
> > the guest.  But that would be unfortunate for the RHEL5-64bit
> > PV guests that actually DO use it.
> Wait, what?  Could you point out where this is in RHEL-5 
> 64-bit PV?  The only
> case of write_tsc() I see in the code is in 
> arch/i386/kernel/smpboot.c, which is
> not used by the Xen PV implementation in RHEL-5.  Where else in the PV
> implementation does a write_tsc?

Hi Chris --

I was surprised also, and digging deeper it looks like I was mistaken.

I instrumented a hypervisor so that Xen would printk a console
message if it was ignoring a wrmsr and was getting output
when I launched a RHEL-5 PV guest.  But I refined the
printk and it is NOT wrmsr(0x10) so you're right, it is
NOT a write_tsc.

Thanks for pointing out my error.

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