lkml.org 
[lkml]   [2020]   [May]   [31]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
Date
SubjectRe: [PATCH] sh: Implement __get_user_u64() required for 64-bit get_user()
Hi Adrian,

On Sun, May 31, 2020 at 11:59 AM John Paul Adrian Glaubitz
<glaubitz@physik.fu-berlin.de> wrote:
> On 5/31/20 11:54 AM, John Paul Adrian Glaubitz wrote:
> > On 5/31/20 11:52 AM, Geert Uytterhoeven wrote:
> >> As this is the 64-bit variant, I think this single move should be
> >> replaced by a double move:
> >>
> >> "mov #0,%R1\n\t" \
> >> "mov #0,%S1\n\t" \
> >>
> >> Same for the big endian version below.
> >>
> >> Disclaimer: uncompiled, untested, no SH assembler expert.
> >
> > Right, this makes sense. I'll send a new patch shortly.
>
> Hmm, this change is not the case for __put_user_asm() vs. __put_user_u64().
> But I have to admit, I don't know what the part below "3:\n\t" is for.

It's part of the exception handling, in case the passed (userspace) pointer
points to an inaccessible address, and triggers an exception.

For an invalid store, nothing is done, besides returning -EFAULT.
Hence there's no "mov #0, %1\n\t" in the put_user case.
For an invalid load, the data is replaced by zero, and -EFAULT is returned.

> +__asm__ __volatile__( \
> + "1:\n\t" \
> + "mov.l %2,%R1\n\t" \
> + "mov.l %T2,%S1\n\t" \
> + "2:\n" \

(reordering the two sections for easier explanation)

> + ".section __ex_table,\"a\"\n\t" \
> + ".long 1b, 3b\n\t" \

In case an exception happens for the instruction at 1b, jump to 3b.

Note that the m68k version has two entries here: one for each half of
the 64-bit access[*].
I don't know if that is really needed (and thus SH needs it, too), or if
the exception code handles subsequent instructions automatically.

> + ".section .fixup,\"ax\"\n" \
> + "3:\n\t" \
> + "mov #0, %1\n\t" \

Return zero instead of the data at the (invalid) address.

> + "mov.l 4f, %0\n\t" \
> + "jmp @%0\n\t" \

Resume at 2b.
Remember: branch delay slot, so the instruction below is executed first!

> + " mov %3, %0\n\t" \

Set err to -EFAULT.

> + ".balign 4\n" \
> + "4: .long 2b\n\t" \
> + ".previous\n" \

> + ".previous" \
> + :"=&r" (err), "=&r" (x) \
> + :"m" (__m(addr)), "i" (-EFAULT), "0" (err)); })

[*] arch/m68k/include/asm/uaccess_mm.h

"1: "MOVES".l (%2)+,%1\n" \
"2: "MOVES".l (%2),%R1\n" \

" .section __ex_table,\"a\"\n" \
" .align 4\n" \
" .long 1b,10b\n" \
" .long 2b,10b\n" \

Gr{oetje,eeting}s,

Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds

\
 
 \ /
  Last update: 2020-05-31 12:44    [W:0.050 / U:6.200 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site