Re: [PATCH] prctl.2: add information regarding PR_SET_FP_MODE and PR_GET_FP_MODE options

["Michael Kerrisk (man-pages)" <mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

Hi Eugene,

I have applied this patch and tweaked some pieces (and pushed to Git),
but some parts need work still, I think. See my comments below.

Rather than sending a patch, I think it would be easier if you just
respond to my comments in line below.


On 11/20/2016 01:55 AM, Eugene Syromyatnikov wrote:
> Based on description provided in commit 9791554b and information in
> https://dmz-portal.mips.com/wiki/MIPS_O32_ABI_-_FR0_and_FR1_Interlinking
> ---
>  man2/prctl.2 | 100 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 100 insertions(+)
> 
> diff --git a/man2/prctl.2 b/man2/prctl.2
> index ae1fbc5..954641c 100644
> --- a/man2/prctl.2
> +++ b/man2/prctl.2
> @@ -263,6 +263,98 @@ or
>  Return the endian-ness of the calling process,
>  in the location pointed to by
>  .IR "(int\ *) arg2" .
> +.\" 9791554b45a2acc28247f66a5fd5bbc212a6b8c8
> +.TP
> +.BR PR_SET_FP_MODE " (since Linux 4.0, only on MIPS)"
> +On MIPS, user land code can be built using ABI which permits linking with a code
> +with a more restrictive floating point requirements.  For example, user land
> +code may be built to target the O32 FPXX ABI and linked with code built for
> +either one of the more restrictive FP32 or FP64. When more restrictive code is
> +linked in, the overall requirement for the process is to use this more
> +restrictive floating point mode. Since kernel has no means of knowing in advance
> +which mode process should be executed in, and having possibility that these
> +restrictions can be changed during the process' lifetime, 

I have trouble to understand "and having possibility that these
restrictions can be changed during the process' lifetime".

I think the main problem is: who can do the changing?
It's not clear from the sentence. Can you explain a little?

> the ability to control
> +it from the user space via this option is provided.

Does "it" in the previous line mean "the floating-point mode"?

> +
> +.\" https://dmz-portal.mips.com/wiki/MIPS_O32_ABI_-_FR0_and_FR1_Interlinking
> +The
> +.I (unsigned int) arg2
> +argument is a bit mask describing floating point mode used:
> +.RS
> +.TP
> +.BR PR_FP_MODE_FR " = " "(1 << 0)"

I removed the piece [" = " "(1 << 0)"] (and similar below), since
presumably user-space doesn't need to know the precise numeric
values.

> +When this bit is
> +.I unset
> +(so called
> +.BR FR=0 " or " FR0
> +mode), 32 FP registers are 32-bit wide, and 64-bit registers are represented as

What are "32 FP registers"? (And again below.)

Perhaps this is just a language issue. Should it be "the 32 FP registers"?
(The lack of an article--"the" or "a"--can sometimes be fatal to 
comprehension in English.)

> +pair of registers (even- and odd- numbered, with even-numbered register
> +containing lower 32 bits, and odd-numbered register containing higher 32 bits).
> +When this bit is
> +.I set
> +(on supported hardware), 32 FP registers are 64-bit wide (so called

"the 32 FP registers"?

> +.BR FR=1 " or " FR1
> +mode). Note that modern MIPS implementations (MIPS R6 and newer) support
> +.B FR=1
> +mode only.
> +
> +Applications that use O32 FP32 ABI can operate only when this bit is
> +.I unset
> +.RB ( FR=0 ;
> +or they can be used with FRE enabled, see below).
> +Applications that use O32 FP64 ABI (and O32 FP64A ABI, which exists for
> +providing ability to operate with existing FP32 code; see below) can operate
> +only when this bit is
> +.I set
> +.RB ( FR=1 ).
> +Applications that use O32 FPXX ABI can operate in both cases.

What does "both cases" mean in the above line?

> +.TP
> +.BR PR_FP_MODE_FRE " = " "(1 << 1)"
> +Compatibility with 32-bit FP mode. When this mode is enabled, it emulates 32-bit

I think the first sentence needs at least one or two more words.  Is

    Provide compatibility with 32-bit FP mode.

correct?

> +FP operations by raising reserved instruction exception on every instruction
> +that uses 32-bit formats and kernel then handles the instruction in software
> +(the problem lies in discrepancy of handling odd-numbered registers which are
> +high 32 bits of 64-bit registers with even numbers in
> +.B FR=0
> +mode and lower 32-bit parts of odd-numbered 64-bit registers in
> +.B FR=1
> +mode). Enabling of this bit is needed when code with O32 FP32 ABI should operate
> +with code with compatible O32 FPXX or O32 FP64A ABIs (which require
> +.B FR=1
> +FPU mode) or when it is executed on newer hardware (MIPS R6 onwards) which lacks
> +.B FR=0
> +mode support when binary with FP32 ABI is used.
> +.IP
> +Note that this mode only makes sense when FPU is in 64-bit mode
> +.RB ( FR=1 .)
> +.IP
> +Note that usage of emulation inherently has a significant performance hit and
> +should be avoided if possible.
> +.RE
> +.IP
> +Note that for N32/N64 ABI is a different story and does not need FPU emulation
> +and always operates in
> +.B FR=1
> +mode.

That last sentence is a bit hard to parse. Would the following be okay:

[[
For the N32/N64 ABI, the story is different story: FPU emulation
is not required and the FPU always operates in
.B FR=1
mode.
]]

?

> +.IP
> +This option is mainly intended for use by dynamic loader, but may be of use by
> +applications in case library loading during runtime (via
> +.BR dlopen (3),
> +for example) is used.
> +.IP
> +Arguments
> +.IR arg3 ", " arg4 " and " arg5
> +are ignored.
> +.TP
> +.BR PR_GET_FP_MODE " (since Linux 4.0, only on MIPS)"
> +Get current floating point mode (see description for
> +.B PR_SET_FP_MODE
> +for details).
> +
> +Call returns bit mask which represents current FP mode in case of success.
> +Arguments
> +.IR arg2 ", " arg3 ", " arg4 " and " arg5
> +are ignored.
>  .TP
>  .BR PR_SET_FPEMU " (since Linux 2.4.18, 2.5.9, only on ia64)"
>  Set floating-point emulation control bits to \fIarg2\fP.
> @@ -1285,6 +1377,14 @@ or
>  and the kernel or the CPU does not support MPX management.
>  Check that the kernel and processor have MPX support.
>  .TP
> +.B EOPNOTSUPP
> +.I option
> +is
> +.B PR_SET_FP_MODE
> +and
> +.I arg2
> +has invalid or unsupported value.
> +.TP
>  .B EPERM
>  .I option
>  is

Cheers,

Michael


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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