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Date:	Tue, 07 Oct 2014 13:03:32 -0700
From:	David Daney <ddaney.cavm@...il.com>
To:	Andy Lutomirski <luto@...capital.net>
CC:	Rich Felker <dalias@...c.org>,
	Leonid Yegoshin <Leonid.Yegoshin@...tec.com>,
	Matthew Fortune <Matthew.Fortune@...tec.com>,
	David Daney <david.s.daney@...il.com>,
	David Daney <ddaney.cavm@...il.com>,
	"libc-alpha@...rceware.org" <libc-alpha@...rceware.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mips@...ux-mips.org" <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 10/07/2014 12:28 PM, Andy Lutomirski wrote:
> On Tue, Oct 7, 2014 at 12:21 PM, Rich Felker <dalias@...c.org> wrote:
>> On Tue, Oct 07, 2014 at 12:16:59PM -0700, Leonid Yegoshin wrote:
>>> On 10/07/2014 12:09 PM, Rich Felker wrote:
>>>> I agree completely here. We should not break things (or, as it
>>>> seems, leave them broken) for common usage cases that affect
>>>> everyone just to coddle proprietary vendor-specific instructions.
>>>> The latter just should not be used in delay slots unless the chip
>>>> vendor also promises to provide fpu branch in hardware. Rich
>>> And what do you propose - remove a current in-stack emulation and
>>> you still think it doesn't break a status-quo?
>>
>> The in-stack trampoline support could be left but used only for
>> emulating instructions the kernel doesn't know. This would make all
>> normal binaries immediately usable with non-executable stack, and
>> would avoid the only potential source of regressions. Ultimately I
>> think the "xol" stuff should be removed, but that could be a long term
>> goal.
>
> Does anything break if the xol stuff is disabled for PT_GNU_STACK tasks?
>

The instructions must be executed, if you turn on a non-executable 
stack, you cannot execute them on the stack, so they must be handled in 
another way, which is the subject of this thread.

Options:

1a) XOL kernel manages the memory
1b) XOL userspace manages the menory
2) Emulate the instructions.
3) I don't think there is a 3rd. option.

As the imgtec people have said, you have to do #2 for their new r6 ISA, 
as it uses PC relative instructions.

I really think we should bite the bullet and do #2 for everything, it 
will be the cleanest long term solutions.

David Daney
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