linux-kernel - Re: [RFC PATCH 1/2] thread_local_abi system call: caching current CPU number (x86)

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20151213181527.GV15533@two.firstfloor.org>
Date:	Sun, 13 Dec 2015 19:15:27 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org,
	Paul Turner <pjt@...gle.com>, Andrew Hunter <ahh@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Andy Lutomirski <luto@...capital.net>,
	Andi Kleen <andi@...stfloor.org>,
	Dave Watson <davejwatson@...com>, Chris Lameter <cl@...ux.com>,
	Ingo Molnar <mingo@...hat.com>, Ben Maurer <bmaurer@...com>,
	Steven Rostedt <rostedt@...dmis.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Josh Triplett <josh@...htriplett.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-api@...r.kernel.org
Subject: Re: [RFC PATCH 1/2] thread_local_abi system call: caching current
 CPU number (x86)

> This getcpu cache is an alternative to the sched_getcpu() vdso which has
> a few benefits:


Note the first version of getcpu() I proposed had a cache. But it was
rejected.

> - It is faster to do a memory read that to call a vDSO,
> - This cached value can be read from within an inline assembly, which
>   makes it a useful building block for restartable sequences.

On x86 we already have the de-facto ABI of using LSL with the magic
segment directly. While that is a few cycles slower than a memory load
I question the difference is big enough to justify a new system call,
and risk slow page fault in context switches.

BTW the vdso could be also optimized I think. For example glibc today
does some stupid (slow) things with it, like doing double iindirect
jumps.

-Andi
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/