linux-kernel - Re: [PATCH 07/12] x86/entry/64: Always run ptregs-using syscalls on the slow path

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Message-ID: <20151208185608.GA3004@gmail.com>
Date:	Tue, 8 Dec 2015 19:56:08 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Brian Gerst <brgerst@...il.com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Andy Lutomirski <luto@...nel.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Borislav Petkov <bp@...en8.de>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 07/12] x86/entry/64: Always run ptregs-using syscalls on
 the slow path


* Brian Gerst <brgerst@...il.com> wrote:

> > We could adjust it a bit and check whether we're in C land (by checking rsp 
> > for ts) and jump into the slow path if we aren't, but I'm not sure this is a 
> > huge win.  It does save some rodata space by avoiding duplicating the table.
> 
> The syscall table is huge.  545*8 bytes, over a full page. Duplicating it for 
> just a few different entries is wasteful.

Note that what matters more is cache footprint, not pure size: 1K of RAM overhead 
for something as fundamental as system calls is trivial cost.

So the questions to ask are along these lines:

 - what is the typical locality of access (do syscall numbers cluster in time and 
   space)

 - how frequently would the two tables be accessed (is one accessed less 
   frequently than the other?)

 - subsequently how does the effective cache footprint change with the 
   duplication?

it might still end up not being worth it - but it's not the RAM cost that is the 
main factor IMHO.

Thanks,

	Ingo
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