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RE: [Xen-devel] [PATCH] replace rdtsc emulation-vs-native xen boot option with per-domain (hypervisor part)
Hi Jeremy --
I think it's fair for us both to completely air our opinions,
but since you've contradicted some of my "hand-wavy" points
and since much of your position is based on the contradictions,
I feel it's necessary to respond.
> The TSC is not, and has never been reliable.
Your data is stale. Please discuss this with processor
and system vendors (I have) and look at the latest upstream
I agree that this presumed reliability should be carefully
tested. (See my proposed patch posted yesterday.) If
you are correct and I am not, your position (that "all
apps that use rdtsc are fundamentally broken") is much
> Except that it comes with a terrible cost...
> This is a massive regression...
It is certainly significant but "terrible" and "massive"
are a bit strong. Based on my measurements, the examples
you cite will degrade performance by a fraction of a percent.
And this loss can be eliminated IF the administrator
understands the risks and chooses performance over
correctness by specifying an option at guest creation.
> The fact that you haven't named a single real app...
> Are you really arguing on the basis that "some apps
> might use tsc in a fragile way" or do you actually have a
> specific list
I have a (small) specific list. For various reasons,
I cannot go into further detail.
But if TSC is indeed reliable on newer processors/systems
(see above) AND on VMware, there will certainly be more.
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