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

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Date:	Wed, 22 Oct 2008 15:00:46 -0700
From:	Alok Kataria <akataria@...are.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	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.

[dropped LKML of the cc list]

Hi Andi,

On Wed, 2008-10-22 at 12:58 -0700, Andi Kleen wrote:
> > We also force set the CONSTANT_TSC capability bit if we are running on
> > VMware, since the VMware hypervisor exports a constant TSC to the guest.
> 
> NAK.
> 
> The whole point of this is to eliminate the need for vmware detection,
> which is something the kernel shouldn't need to know about.
> Instead VMware should supply that bit by itself and then Linux could
> do the right thing. The only change you would need to do is to always
> check it for all CPU vendors, not just AMD.

Yeah, i can add that code which checks for that bit, for all cpu
vendors. And yes we can supply that bit with our future products.

> Adding it with vmware detection code is not useful.

The VMware detection code is something that's needed anyways for other
purposes(getting tsc_freq right now). So the code which force sets this
TSC_CONSTANT bit is trivial.
Apart from that, as i said yesterday, this should be viewed as a special
case for VMware products which don't already set this bit. Changing the
behavior for already existing products is not feasible.

Thanks,
Alok

> 
> -Andi

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