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Re: [PATCH resend] MIPS: Allow FPU emulator to use non-stack area.
- From: Rich Felker <dalias at libc dot org>
- To: Matthew Fortune <Matthew dot Fortune at imgtec dot com>
- Cc: David Daney <david dot s dot daney at gmail dot com>, David Daney <ddaney at caviumnetworks dot com>, Andy Lutomirski <luto at amacapital dot net>, David Daney <ddaney dot cavm at gmail dot com>, "libc-alpha at sourceware dot org" <libc-alpha at sourceware dot org>, "linux-kernel at vger dot kernel dot org" <linux-kernel at vger dot kernel dot org>, "linux-mips at linux-mips dot org" <linux-mips at linux-mips dot org>, David Daney <david dot daney at cavium dot com>, Leonid Yegoshin <Leonid dot Yegoshin at imgtec dot com>
- Date: Tue, 7 Oct 2014 07:19:00 -0400
- Subject: Re: [PATCH resend] MIPS: Allow FPU emulator to use non-stack area.
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- References: <20141006213101 dot GA23797 at brightrain dot aerifal dot cx> <54330D79 dot 80102 at caviumnetworks dot com> <20141006215813 dot GB23797 at brightrain dot aerifal dot cx> <543327E7 dot 4020608 at amacapital dot net> <54332A64 dot 5020605 at caviumnetworks dot com> <20141007000514 dot GD23797 at brightrain dot aerifal dot cx> <543334CE dot 8060305 at caviumnetworks dot com> <20141007004915 dot GF23797 at brightrain dot aerifal dot cx> <54337127 dot 40806 at gmail dot com> <6D39441BF12EF246A7ABCE6654B0235320F1E173 at LEMAIL01 dot le dot imgtec dot 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