linux-kernel - Re: [PATCH v4 1/5] getcpu_cache system call: cache CPU number of running thread

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Message-ID: <1401667361.10273.1456617236327.JavaMail.zimbra@efficios.com>
Date:	Sat, 27 Feb 2016 23:53:56 +0000 (UTC)
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Russell King <linux@....linux.org.uk>,
	Ingo Molnar <mingo@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-api <linux-api@...r.kernel.org>,
	Paul Turner <pjt@...gle.com>, Andrew Hunter <ahh@...gle.com>,
	Andy Lutomirski <luto@...capital.net>,
	Andi Kleen <andi@...stfloor.org>,
	Dave Watson <davejwatson@...com>, Chris Lameter <cl@...ux.com>,
	Ben Maurer <bmaurer@...com>, rostedt <rostedt@...dmis.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Josh Triplett <josh@...htriplett.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Michael Kerrisk <mtk.manpages@...il.com>
Subject: Re: [PATCH v4 1/5] getcpu_cache system call: cache CPU number of
 running thread

----- On Feb 27, 2016, at 1:35 PM, Linus Torvalds torvalds@...ux-foundation.org wrote:

> On Sat, Feb 27, 2016 at 6:58 AM, Peter Zijlstra <peterz@...radead.org> wrote:
>>
>> Paul's patches have the following structure:
>>
>> struct thread_local_abi {
>>         union {
>>                 struct {
>>                         u32     cpu_id;
>>                         u32     seq;
>>                 };
>>                 u64 cpu_seq;
>>         };
>>         unsigned long post_commit_ip;
>> };
> 
> Please don't do "unsigned long" in ABI structures any more.
> 
> Make it u64, and make sure it is 64-bit aligned (which it would be in
> this case). Make it so that we don't have to have separate compat
> paths.

AFAIU, this "post_commit_ip" field is expected to be updated
with a single-copy-store by user-space. If we want to handle both
32-bit and 64-bit processes, how do you recommend doing this
without an unsigned long type ?

A 64-bit integer would not be a single-copy store for
32-bit processes, but a 32-bit integer would not be large
enough for 64-bit processes.

Would a

union {
    uint32_t val32;
    uint64_t val64;
} field;

be an acceptable option ? Then the kernel could use
one field or the other depending on the process bitness.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com