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

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Date:	Tue, 7 Oct 2014 09:13:22 +0000
From:	Matthew Fortune <Matthew.Fortune@...tec.com>
To:	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.

> >> 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].

Matthew
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