linux-kernel - Re: [PATCH] powerpc: slightly improve cache helpers

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]

Date:   Tue, 7 May 2019 18:53:30 +0200
From:   Christophe Leroy <christophe.leroy@....fr>
To:     Segher Boessenkool <segher@...nel.crashing.org>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH] powerpc: slightly improve cache helpers



Le 07/05/2019 à 17:10, Segher Boessenkool a écrit :
> Hi Christophe,
> 
> On Tue, May 07, 2019 at 01:31:39PM +0000, Christophe Leroy wrote:
>> Cache instructions (dcbz, dcbi, dcbf and dcbst) take two registers
>> that are summed to obtain the target address. Using '%y0' argument
>> gives GCC the opportunity to use both registers instead of only one
>> with the second being forced to 0.
> 
> That's not quite right.  Sorry if I didn't explain it properly.
> 
> "m" allows all memory.  But this instruction only allows reg,reg and
> 0,reg addressing.  For that you need to use constraint "Z".

But gcc help 
(https://gcc.gnu.org/onlinedocs/gcc/Machine-Constraints.html#Machine-Constraints) 
says it is better to use 'm':

Z

     Memory operand that is an indexed or indirect from a register (it 
is usually better to use ‘m’ or ‘es’ in asm statements)

That's the reason why I used 'm', I thought it was equivalent.

Christophe

> 
> The output modifier "%y0" just makes [reg] (i.e. simple indirect addressing)
> print as "0,reg" instead of "0(reg)" as it would by default (for just "%0").
> 
> 
> Segher
>