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

To: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>, Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Subject: Re: [Xen-devel] write_tsc in a PV domain?
From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Date: Wed, 26 Aug 2009 07:23:54 +0100
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On 26/08/2009 00:09, "Dan Magenheimer" <dan.magenheimer@xxxxxxxxxx> wrote:

>> arch/x86/traps.c:emulate_privileged_op(), case 0x30.  It looks like
>> writing to 0x10 would be silently ignored.
> Hmmm... maybe I am misreading the code but it looks like the
> default case will end up with "goto fail" which will not
> update IP and so will infinite loop trapping on that instruction.
> It appears that write_tsc calls are made in linux-2.6.18 (though
> apparently never get executed) but disappear somewhere before
> 2.6.24 and don't exist in 2.6.30 either.  So perhaps write_tsc
> has never been executed in a PV guest and just doesn't work.

Jeremy is correct. The TSC MSR cannot be written. Most that will happen is
that Xen will print a warning message, but the WRMSR instruction will always
be skipped over.

 -- Keir

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