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

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Date:	Tue, 7 Oct 2014 19:18:33 -0500
From:	Chuck Ebbert <cebbert.lkml@...il.com>
To:	David Daney <ddaney@...iumnetworks.com>
Cc:	Ralf Baechle <ralf@...ux-mips.org>, Rich Felker <dalias@...c.org>,
	David Daney <ddaney.cavm@...il.com>,
	<libc-alpha@...rceware.org>, <linux-kernel@...r.kernel.org>,
	<linux-mips@...ux-mips.org>, David Daney <david.daney@...ium.com>
Subject: Re: [PATCH resend] MIPS: Allow FPU emulator to use non-stack area.

On Tue, 7 Oct 2014 16:59:03 -0700
David Daney <ddaney@...iumnetworks.com> wrote:

> On 10/07/2014 04:20 PM, Ralf Baechle wrote:
> > On Mon, Oct 06, 2014 at 02:18:19PM -0700, David Daney wrote:
> >
> >>> As an alternative, if the space of possible instruction with a delay
> >>> slot is sufficiently small, all such instructions could be mapped as
> >>> immutable code in a shared mapping, each at a fixed offset in the
> >>> mapping. I suspect this would be borderline-impractical (multiple
> >>> megabytes?), but it is the cleanest solution otherwise.
> >>>
> >>
> >> Yes, there are 2^32 possible instructions.  Each one is 4 bytes, plus you
> >> need a way to exit after the instruction has executed, which would require
> >> another instruction.  So you would need 32GB of memory to hold all those
> >> instructions, larger than the 32-bit virtual address space.
> >
> > Plus errata support for some older CPUs requires no other instructions
> > that might cause an exception to be present in the same cache line inflating
> > the size to 32 bytes per instruction.
> >
> > I've contemplated a full emulation - but that would require an emulator that
> > is capable of most of the instruction set.  With all the random ASEs around
> > that would be hard to implement while the FPU emulator trampoline as currently
> > used has the advantage of automatically supporting ASEs, known and unknown.
> > So it's a huge bonus for maintenance.
> >
> 
> Unfortunatly it breaks when our friends at Imgtec introduce their PC 
> relative instructions in mipsr6, so an emulator may be unavoidable.
> 

The x86 kprobes code deals with executing relocated insns with
PC-relative offsets by adjusting the offset in a relocated instruction
before executing it.

See arch/x86/kernel/kprobes/core.c::__copy_instruction()
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