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

[James Hogan <james.hogan@imgtec.com>]

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