[coreboot] [patch]: memcpy/memset inline asm & config_compress disabled when needed

[Marc Jones]

A couple of thoughts. Can we move the data as dwords instead of bytes
for uncompressed data? As for the cache, can you check the MTRRs and
CR0 before the copy? We should cache the cbfs instructions and data
being moved. Does the cbfs code need a call to setup the cache with
the source or should it be setup prior to calling cbfs?

Marc


On Mon, Feb 22, 2010 at 6:45 PM, Bao, Zheng <Zheng.Bao at amd.com> wrote:
> I think the memcpy/memset and decompressing are slow because of the Icache. Only one instruction executes repeatedly in the asm code. It doesn't have to access the instructions in the ROM every time. The memcpy/memset are easy to narrow down to a single instruction. But ulzma() can not. So I don't like my patch either. Does anybody have the idea to finally solve this problem?
>
> Zheng
>
> -----Original Message-----
> From: ron minnich [mailto:rminnich at gmail.com]
> Sent: Tuesday, February 23, 2010 12:29 AM
> To: Bao, Zheng
> Cc: coreboot at coreboot.org
> Subject: Re: [coreboot] [patch]: memcpy/memset inline asm & config_compress disabled when needed
>
> On Sun, Feb 21, 2010 at 8:48 PM, Bao, Zheng <Zheng.Bao at amd.com> wrote:
>
>
>>
>> Index: src/lib/memcpy.c
>> ===================================================================
>> --- src/lib/memcpy.c    (revision 5133)
>> +++ src/lib/memcpy.c    (working copy)
>> @@ -3,10 +3,14 @@
>>  {
>>        const char *src = vsrc;
>>        char *dest = vdest;
>> -       int i;
>>
>> -       for (i = 0; i < (int)bytes; i++)
>> -               dest[i] = src[i];
>> +       __asm__ __volatile__ (                          \
>> +               "cld \n\t"                              \
>> +               "rep \n\t"                              \
>> +               "movsb"                                 \
>> +               :               /* No output */         \
>> +               : "S"(src), "D"(dest), "c"(bytes)       \
>> +               );
>>
>>        return vdest;
>>  }
>> Index: src/lib/memset.c
>> ===================================================================
>> --- src/lib/memset.c    (revision 5133)
>> +++ src/lib/memset.c    (working copy)
>> @@ -2,11 +2,15 @@
>>
>>  void *memset(void *s, int c, size_t n)
>>  {
>> -       int i;
>>        char *ss = (char *) s;
>>
>> -       for (i = 0; i < (int)n; i++)
>> -               ss[i] = c;
>> +       __asm__ __volatile__ (                  \
>> +               "cld\n\t"                       \
>> +               "rep\n\t"                       \
>> +               "stosb"                         \
>> +               :                               \
>> +               : "a"(c), "D"(ss), "c"(n)       \
>> +               );
>>
>>        return s;
>>  }
>
> I'm glad this works, but I am afraid I have a concern about it.
>
> I've become opposed to inline assembly on several principles in the
> last few years:
>
> - we've seen one problem after another as the manner in which __asm__
> is supposed to be used has varied as gcc changes.
>  we've also had code that had errors in __asm__ which nobody noticed.
>
> - I'd like to understand why the C code is so much worse. I can't see
> a good reason for C to be so bad at this simple task.
> For example, with the memset, what if we just created a memzero
> function instead, which used a constant '0' instead of a parameter:
> does the code improve?
>
> - I now feel a .c file should contain C. If we need a .s, let's create a .s.
>
> So, first, can we please have a look at the .s produced by the C code
> and see if we can understand why it is so slow?
>
> Thanks
>
> Ron
>
>
>
> --
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>



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