linux-kernel - Re: [PATCH resend] MIPS: Allow FPU emulator to use non-stack area.

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Date:	Mon, 6 Oct 2014 17:29:41 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Andrew Pinski <pinskia@...il.com>
Cc:	Rich Felker <dalias@...c.org>,
	David Daney <ddaney@...iumnetworks.com>,
	David Daney <ddaney.cavm@...il.com>,
	GNU C Library <libc-alpha@...rceware.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux MIPS Mailing List <linux-mips@...ux-mips.org>,
	David Daney <david.daney@...ium.com>
Subject: Re: [PATCH resend] MIPS: Allow FPU emulator to use non-stack area.

On Mon, Oct 6, 2014 at 5:28 PM, Andrew Pinski <pinskia@...il.com> wrote:
> On Mon, Oct 6, 2014 at 5:21 PM, Rich Felker <dalias@...c.org> wrote:
>> On Mon, Oct 06, 2014 at 05:11:38PM -0700, Andrew Pinski wrote:
>>> On Mon, Oct 6, 2014 at 5:05 PM, Rich Felker <dalias@...c.org> wrote:
>>> > On Mon, Oct 06, 2014 at 04:48:52PM -0700, David Daney wrote:
>>> >> On 10/06/2014 04:38 PM, Andy Lutomirski wrote:
>>> >> >On 10/06/2014 02:58 PM, Rich Felker wrote:
>>> >> >>On Mon, Oct 06, 2014 at 02:45:29PM -0700, David Daney wrote:
>>> >> [...]
>>> >> >>This is a huge ill-designed mess.
>>> >> >
>>> >> >Amen.
>>> >> >
>>> >> >Can the kernel not just emulate the instructions directly?
>>> >>
>>> >> In theory it could, but since there can be implementation defined
>>> >> instructions, there is no way to achieve full instruction set
>>> >> coverage for all possible machines.
>>> >
>>> > Is the issue really implementation-defined instructions with delay
>>> > slots? If so it sounds like a made-up issue. They're not going to
>>> > occur in real binaries. Certainly a compiler is not going to generate
>>> > implementation-defined instructions, and if you're writing the asm by
>>> > hand, you just don't put floating point instructions in the delay
>>> > slot.
>>>
>>> It is not the instruction with delay slot but rather the instruction
>>> in the delay slot itself.
>>
>> An instruction in the delay slot for the instruction being emulated?
>> How would that arise? Are there floating point instructions with delay
>> slots?
>
> Yes branches.

I admit I have no idea what's going here, but I find it hard to
believe that having the kernel fix this up for new code is desirable.
Unless MIPS can round-trip a trap *very* quickly, performance will be
awful for any code that has this problem.

--Andy
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