Subject: Re: [PATCH] Skip tsc synchronization checks if CONSTANT_TSC bit is
	set.
From: Alok Kataria <akataria@vmware.com>
Reply-To: akataria@vmware.com
To: Andi Kleen <andi@firstfloor.org>
Cc: Ingo Molnar <mingo@elte.hu>, "H. Peter Anvin" <hpa@zytor.com>,
       LKML <linux-kernel@vger.kernel.org>,
       the arch/x86 maintainers <x86@kernel.org>,
       Daniel Hecht <dhecht@vmware.com>
In-Reply-To: <20081022195845.GP12825@one.firstfloor.org>
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	 <874p36fflp.fsf@basil.nowhere.org> <1224607284.6161.22.camel@alok-dev1>
	 <20081021174008.GH12825@one.firstfloor.org>
	 <1224612294.6161.43.camel@alok-dev1>
	 <20081021181536.GI12825@one.firstfloor.org>
	 <1224616236.6161.60.camel@alok-dev1>
	 <20081021192746.GJ12825@one.firstfloor.org>
	 <1224703427.13953.8.camel@alok-dev1>
	 <20081022195845.GP12825@one.firstfloor.org>
Content-Type: text/plain
Organization: VMware INC.
Date: Wed, 22 Oct 2008 15:00:46 -0700
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[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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