linux-kernel - Re: [PATCH v5 8/9] x86-64: Emulate legacy vsyscalls

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Date:	Tue, 7 Jun 2011 10:30:59 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	pageexec@...email.hu
Cc:	Andrew Lutomirski <luto@....edu>, x86@...nel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org, Jesper Juhl <jj@...osbits.net>,
	Borislav Petkov <bp@...en8.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Jan Beulich <JBeulich@...ell.com>,
	richard -rw- weinberger <richard.weinberger@...il.com>,
	Mikael Pettersson <mikpe@...uu.se>,
	Andi Kleen <andi@...stfloor.org>,
	Brian Gerst <brgerst@...il.com>,
	Louis Rilling <Louis.Rilling@...labs.com>,
	Valdis.Kletnieks@...edu
Subject: Re: [PATCH v5 8/9] x86-64: Emulate legacy vsyscalls


* pageexec@...email.hu <pageexec@...email.hu> wrote:

> > A fastpath is defined by optimization considerations applied to a 
> > codepath (the priority it gets compared to other codepaths), 
> > *not* by its absolute performance.
> 
> we're not talking about random arbitrarily defined paths here but 
> the impact of putting well predicted branches into the pf handler 
> vs. int xx (are you perhaps confused by 'fast path' vs. 
> 'fastpath'?).

So please educate me, what is the difference between 'fast path' 
versus 'fastpath', as used by kernel developers, beyond the space?

> that impact only matters if it's measurable. you have yet to show 
> that it is. and all this sillyness is for a hypothetical situation 
> since those conditional branches don't even need to be in the 
> general page fault processing paths.

Is this some sort of sick joke?

Do you *really* claim that the number of instructions executed in a 
fastpath do not matter and that our years-long effort to shave off an 
instruction here and there from the x86 do_page_fault() code were 
meaningless and that we can add branches with zero cost?

Thanks,

	Ingo
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