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Re: [Xen-devel] [PATCH] x86: don't write_tsc() non-zero values on CPUs u

To: Jan Beulich <JBeulich@xxxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH] x86: don't write_tsc() non-zero values on CPUs updating only the lower 32 bits
From: Keir Fraser <keir.xen@xxxxxxxxx>
Date: Thu, 14 Apr 2011 10:18:18 +0100
Cc: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, "winston.l.wang" <winston.l.wang@xxxxxxxxx>
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Thread-topic: [Xen-devel] [PATCH] x86: don't write_tsc() non-zero values on CPUs updating only the lower 32 bits
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On 14/04/2011 09:06, "Jan Beulich" <JBeulich@xxxxxxxxxx> wrote:

>> For physically-added CPUs only. Kind of unavoidable, that one: we can only
>> try to do our best in that case. And let's face it, that probably affects
>> exactly zero production users of Xen/x86 right now.
> That latter part I agree to.
> But what are you afraid of? Probing the TSC write shouldn't do any
> harm.

You will end up with a BSP TSC value different than what it would have been
if you had not probed. Since you write back a (slightly) stale TSC value.
Which would increase cross-CPU TSC skew if the platform has done a good sync
job at power-on.

Now, do I personally think this much matters? Not really, since I believe
that direct guest TSC access is a huge non-issue. But it is something that
Dan disagreed on, he did a bunch of work on time management, and the code
as-is does try quite hard to avoid writing TSC if at all possible. I don't
think it's a good idea to change this without feedback from Dan, at least.

> Additionally, did you read the comment immediately preceding
> the probing code? AMD doesn't guarantee the TSC to be writable at
> all.
>>> cstate_restore_tsc() also has no such gating afaics.
>> It is gated on NONSTOP_TSC which is implied by TSC_RELIABLE.
> Ah, yes. But (I think) not architecturally, only by virtue of how
> code is currently structured. If that changes, we'd be back at a
> latent (and quite non-obvious) bug.

Yeah, if we want to continue to try avoiding write_tsc() on TSC_RELIABLE
then we should assert !TSC_RELIABLE on the write_tsc() path in

 -- Keir

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