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

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Message-ID: <49011AD7.7000901@zytor.com>
Date:	Thu, 23 Oct 2008 17:46:15 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andi Kleen <andi@...stfloor.org>
CC:	akataria@...are.com, 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.

Andi Kleen wrote:
>> 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.

That is at least to some degree nonsense, simply because we are all well 
down that particular "slippery slope": we have hardware blacklists and 
whitelists, CPU-specific workarounds, and so on all over the place, and 
in that sense a hypervisor really isn't different than another hardware 
platform.

	-hpa
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