linux-kernel - Re: [PATCH] Skip tsc synchronization checks if CONSTANT

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Message-ID: <20081024002511.GC27492@one.firstfloor.org>
Date:	Fri, 24 Oct 2008 02:25:11 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	akataria@...are.com, Andi Kleen <andi@...stfloor.org>,
	Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	Daniel Hecht <dhecht@...are.com>
Subject: Re: [PATCH] Skip tsc synchronization checks if CONSTANT_TSC bit is set.

> As far as skipping the check, it makes sense for me in the case of known 
> virtualization platforms; a CPU feature bit, real or synthetic, is a 
> very clean way to do that. 

I don't think adding detection for non PV Hypervisors is anywhere clean
Even if it's only VMware today, tomorrow it will be a few more
and long term you might need to support all of the obscuro hypervisors
that are out there. Just seems like a slippery slope. Either it's 
paravirtual or it's not, but it should attempt to be both. If the hypervisor
doesn't emulate TSC well enough that the native code works it's entirely
reasonable to let it use some other timer, like it has been always
done in the past.

-Andi
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