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

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Message-ID: <20081022225409.GB27492@one.firstfloor.org>
Date:	Thu, 23 Oct 2008 00:54:09 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Alok Kataria <akataria@...are.com>
Cc:	Andi Kleen <andi@...stfloor.org>, 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.

> Not really, there are problems with the pm timer too, the one about
> missing the counter wrap and time dropping in chunks of 4 seconds.
> Tried to explain it over here, http://lkml.org/lkml/2008/10/22/525

Well then pit. Or are you saying time is always broken on VMware & Linux?

> So TSC is the ideal clocksource from performance and correctness point
> of view for VMware. 

But you don't seem to emulate it "ideal"ly otherwise you wouldn't
need all these hacks you're adding? 

I think you should either implement a TSC that matches what
real hardware does (including CPUID semantics) or implement
a real vmware PV timer and just say it's PV and not fully virtualized. 
But doesn't the vmware paravirt ops have that already anyways?

But I personally think it wouldn't really scale to add detection for 
more and more "nearly PV" hypervisors to the standard native kernel.

-Andi
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