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

[Rich Felker <dalias@libc.org>]

On Tue, Oct 07, 2014 at 09:13:22AM +0000, Matthew Fortune wrote:
> 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)?

Yes. If a trampoline is being generated to replace the delay slot
instruction, it can just contain more complex code to duplicate what
the PC-relative instruction would have done. Since the ABI already
assumes a stack is available, it can use the stack to backup registers
it needs for scratch space and restore them.

> 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].

Really everything should be done in the kernel, and it's not as hard
as people are making it look. The kernel _already_ has to enforce MM
context permissions for every syscall that reads or writes user memory
(e.g. futex with PI mutexes or FUTEX_WAKE_OP, or even simple things
like read/write) so there's no reason it can't do emulated
loads/stores the exact same way.

Rich