linux-kernel - RE: [PATCH v2 3/6] powerpc: Convert flush_icache

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Message-Id: <b9f8b21afc0f86e9757daf3de2794144ce2565e6.camel@au1.ibm.com>
Date:   Wed, 04 Sep 2019 13:36:08 +1000
From:   "Alastair D'Silva" <alastair@....ibm.com>
To:     Segher Boessenkool <segher@...nel.crashing.org>,
        Christophe Leroy <christophe.leroy@....fr>
Cc:     David Hildenbrand <david@...hat.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, Nicholas Piggin <npiggin@...il.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Paul Mackerras <paulus@...ba.org>, Qian Cai <cai@....pw>,
        Thomas Gleixner <tglx@...utronix.de>,
        linuxppc-dev@...ts.ozlabs.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Allison Randal <allison@...utok.net>
Subject: RE: [PATCH v2 3/6] powerpc: Convert flush_icache_range & friends to C

On Tue, 2019-09-03 at 11:04 -0500, Segher Boessenkool wrote:
> On Tue, Sep 03, 2019 at 04:28:09PM +0200, Christophe Leroy wrote:
> > Le 03/09/2019 à 15:04, Segher Boessenkool a écrit :
> > > On Tue, Sep 03, 2019 at 03:23:57PM +1000, Alastair D'Silva wrote:
> > > > +	asm volatile(
> > > > +		"   mtctr %2;"
> > > > +		"   mtmsr %3;"
> > > > +		"   isync;"
> > > > +		"0: dcbst   0, %0;"
> > > > +		"   addi    %0, %0, %4;"
> > > > +		"   bdnz    0b;"
> > > > +		"   sync;"
> > > > +		"   mtctr %2;"
> > > > +		"1: icbi    0, %1;"
> > > > +		"   addi    %1, %1, %4;"
> > > > +		"   bdnz    1b;"
> > > > +		"   sync;"
> > > > +		"   mtmsr %5;"
> > > > +		"   isync;"
> > > > +		: "+r" (loop1), "+r" (loop2)
> > > > +		: "r" (nb), "r" (msr), "i" (bytes), "r" (msr0)
> > > > +		: "ctr", "memory");
> > > 
> > > This outputs as one huge assembler statement, all on one
> > > line.  That's
> > > going to be fun to read or debug.
> > 
> > Do you mean \n has to be added after the ; ?
> 
> Something like that.  There is no really satisfying way for doing
> huge
> inline asm, and maybe that is a good thing ;-)
> 
> Often people write \n\t at the end of each line of inline asm.  This
> works
> pretty well (but then there are labels, oh joy).
> 
> > > loop1 and/or loop2 can be assigned the same register as msr0 or
> > > nb.  They
> > > need to be made earlyclobbers.  (msr is fine, all of its reads
> > > are before
> > > any writes to loop1 or loop2; and bytes is fine, it's not a
> > > register).
> > 
> > Can you explicit please ? Doesn't '+r' means that they are input
> > and 
> > output at the same time ?
> 
> That is what + means, yes -- that this output is an input as
> well.  It is
> the same to write
> 
>   asm("mov %1,%0 ; mov %0,42" : "+r"(x), "=r"(y));
> or to write
>   asm("mov %1,%0 ; mov %0,42" : "=r"(x), "=r"(y) : "0"(x));
> 
> (So not "at the same time" as in "in the same machine instruction",
> but
> more loosely, as in "in the same inline asm statement").
> 
> > "to be made earlyclobbers", what does this means exactly ? How to
> > do that ?
> 
> You write &, like "+&r" in this case.  It means the machine code
> writes
> to this register before it has consumed all asm inputs (remember, GCC
> does not understand (or even parse!) the assembler string).
> 
> So just
> 
> 		: "+&r" (loop1), "+&r" (loop2)
> 
> will do.  (Why are they separate though?  It could just be one loop
> var).
> 
> 

Thanks, I've updated these.

-- 
Alastair D'Silva
Open Source Developer
Linux Technology Centre, IBM Australia
mob: 0423 762 819