linux-kernel - Re: x86-64: memset()/memcpy() not fully standards compliant

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Message-Id: <4F06DFEF020000780006AC84@nat28.tlf.novell.com>
Date:	Fri, 06 Jan 2012 10:50:07 +0000
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Andi Kleen" <andi@...stfloor.org>
Cc:	<mingo@...e.hu>, <tglx@...utronix.de>,
	<linux-kernel@...r.kernel.org>, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: x86-64: memset()/memcpy() not fully standards compliant

>>> On 06.01.12 at 11:37, Andi Kleen <andi@...stfloor.org> wrote:
>> > You would slow a critical fast path operation down for something
>> > that never happens?!?
>> 
>> It does happen, just (so far) not in-tree. It's a latent problem that's
>> just waiting for someone else to run into. Apart from large bootmem
>> allocations (where not even the latency of the memory clearing
> 
> Modern kernels are bootmem less.

It's not the traditional bootmem implementation anymore, but
alloc_bootmem() et al still exist, and still clear the allocated memory
(in __alloc_memory_core_early()). So there is a code path that can
validly be used (and it is this code path that is presenting one of the
problems with the non-pv-ops Xen kernels, as they're using flatmem
rather than sparsemem since their physical address space is always
fully continuous).

Jan

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