From: Michael Ellerman <mpe@ellerman•id.au>
To: Christophe LEROY <christophe.leroy@c-s•fr>
Cc: Michal Sojka <sojkam1@fel•cvut.cz>,
Scott Wood <scottwood@freescale•com>,
linuxppc-dev@lists•ozlabs.org, Paul Mackerras <paulus@samba•org>
Subject: Re: memcpy regression
Date: Mon, 07 Sep 2015 18:40:49 +1000 [thread overview]
Message-ID: <1441615249.8979.1.camel@ellerman.id.au> (raw)
In-Reply-To: <55ED37D1.3080503@c-s.fr>
On Mon, 2015-09-07 at 09:08 +0200, Christophe LEROY wrote:
> Hi Michael
>
> Le 07/09/2015 03:14, Michael Ellerman a écrit :
> > On Sun, 2015-09-06 at 23:01 +0200, Michal Sojka wrote:
> >> I found the problem. The compiler replaces an assignment with a call to
> >> memcpy. The following patch fixes the problem for me. However, I'm not
> >> sure whether this is the real solution. I guess the compiler is free to
> >> generate a call to memcpy wherever it wants so other compilers or other
> >> optimization levels may need fixes at other places. What do others
> >> think?
> > I think you're right that it's not a good solution, the compiler could generate
> > other calls to memcpy depending on various factors, and people will add new
> > code that causes memcpy to get called and it will break your platform.
> >
> > Christophe, am I right that the problem here is that your new memcpy() doesn't
> > work until later in boot when caches are enabled?
>
> That's right, memset() and memcpy() are for setting/copying data into
> cacheable RAM.
> They are using dczb instruction in order to avoid wasting time loading
> the cacheline with data that will be overwritten.
>
> memset_io() and memcpy_toio() are the functions to use when using not
> cacheable memory.
>
> The issue identified by Michal is in function setup_cpu_spec() which is
> called by identify_cpu(). identify_cpu() is called from early_init().
> In the begining of early_init(), there is (code from Paul in 2005)
>
> /* First zero the BSS -- use memset_io, some platforms don't have
> * caches on yet */
> memset_io((void __iomem *)PTRRELOC(&__bss_start), 0,
> __bss_stop - __bss_start);
>
> It shows that it is already expected that the cache is not active yet
> and standard memset() shall not be used yet. That's the same with memcpy().
Thanks for the explanation.
> I think GCC uses memcpy() in well known situations like initialising
> structures or copying structures.
> Shouldn't we just avoid this kind of actions in the very few early init
> functions ?
Which are the "very few" early init functions? Can you make a list, for 32-bit
and 64-bit? And can we keep it updated over time and not introduce regressions?
cheers
next prev parent reply other threads:[~2015-09-07 8:40 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-09-04 13:33 memcpy regression Michal Sojka
2015-09-04 13:57 ` Christophe LEROY
2015-09-04 14:35 ` Michal Sojka
2015-09-04 18:10 ` christophe leroy
2015-09-04 19:49 ` Michal Sojka
2015-09-05 0:08 ` Michal Sojka
2015-09-06 8:18 ` christophe leroy
2015-09-06 19:05 ` Michal Sojka
2015-09-06 21:01 ` Michal Sojka
2015-09-07 1:14 ` Michael Ellerman
2015-09-07 7:08 ` Christophe LEROY
2015-09-07 8:40 ` Michael Ellerman [this message]
2015-09-07 9:45 ` Michal Sojka
2015-09-07 10:59 ` David Laight
2015-09-08 3:54 ` Michael Ellerman
2015-09-08 8:59 ` David Laight
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