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

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Message-ID: <20141007190943.GM23797@brightrain.aerifal.cx>
Date:	Tue, 7 Oct 2014 15:09:43 -0400
From:	Rich Felker <dalias@...c.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Leonid Yegoshin <Leonid.Yegoshin@...tec.com>,
	Matthew Fortune <Matthew.Fortune@...tec.com>,
	David Daney <david.s.daney@...il.com>,
	David Daney <ddaney@...iumnetworks.com>,
	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>
Subject: Re: [PATCH resend] MIPS: Allow FPU emulator to use non-stack area.

On Tue, Oct 07, 2014 at 11:44:35AM -0700, Andy Lutomirski wrote:
> > 4)  The voice for doing any instruction emulation in kernel - it is not a
> > MIPS business model to force customer to put details of all Coprocessor 2
> > instructions public. We provide an interface and the rest is a customer
> > business. Besides that it is really painful to make a differentiation
> > between Cavium Octeon and some another CPU instructions with the same
> > opcode. On other side, leaving emulation of their instructions to them is
> > not a wise after having some good way doing that multiple years.
> 
> IMO this is all backwards.  If MIPS customers put proprietary
> instructions into their ISA, they leave out the FPU, and they put a
> proprietary insn in a branch delay slot, then I think that they
> deserve a fatal signal.

I agree completely here. We should not break things (or, as it seems,
leave them broken) for common usage cases that affect everyone just to
coddle proprietary vendor-specific instructions. The latter just
should not be used in delay slots unless the chip vendor also promises
to provide fpu branch in hardware.

Rich
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