Re: [PATCH] Skip tsc synchronization checks if CONSTANT_TSC bit isset.

From: Alok Kataria
Date: Wed Oct 22 2008 - 18:00:56 EST

[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.


> -Andi

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