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

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Date:	Tue, 7 Oct 2014 11:52:34 +0100
From:	James Hogan <james.hogan@...tec.com>
To:	Matthew Fortune <Matthew.Fortune@...tec.com>,
	David Daney <david.s.daney@...il.com>,
	Rich Felker <dalias@...c.org>,
	David Daney <ddaney@...iumnetworks.com>
CC:	Andy Lutomirski <luto@...capital.net>,
	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>,
	"Leonid Yegoshin" <Leonid.Yegoshin@...tec.com>
Subject: Re: [PATCH resend] MIPS: Allow FPU emulator to use non-stack area.

On 07/10/14 10:13, Matthew Fortune wrote:
>>>> the out-of-line execution trick, but do it somewhere other than in
>>>> stack memory.
>>> How do you answer Andy Lutomirski's question about what happens when a
>>> signal handler interrupts execution while the program counter is
>>> pointing at this "out-of-line execution" trampoline? This seems like a
>>> show-stopper for using anything other than the stack.
>> It would be nice to support, but not doing so would not be a regression
>> from current behavior.
> 
> It seems appropriate to mention another issue which should be addressed as
> part of the overall FPU emulation work...
> 
> From what I can see the out-of-line execution of delay slot instructions
> will break micromips R3 addiupc, and all MIPS32r6 and MIPS64r6 PC-relative
> instructions (inc load/store) as they will have the wrong base. Is there
> anything in the current set of proposals that can address this (beyond
> adding restrictions to what is ABI allowed in FPU branch delay slots)?
> 
> This is an issue whether the stack is executable or not but does directly
> relate to the topic of FPU emulation.  It sounds like the kernel would not
> be able to emulate a pc-relative load/store even if it was a special case
> as it would not run in the correct MM context? [be gentle, I'm no expert
> in this area].

I think special casing and emulating them in the kernel would work in
these cases, since it'd be a known set of instructions rather than
arbitrary unknown instructions, the kernel needs to read/write safely
into the user address space all the time for system calls.

Cheers
James
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