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

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Date:	Tue, 7 Oct 2014 11:32:58 -0700
From:	Leonid Yegoshin <Leonid.Yegoshin@...tec.com>
To:	Matthew Fortune <Matthew.Fortune@...tec.com>
CC:	David Daney <david.s.daney@...il.com>,
	Rich Felker <dalias@...c.org>,
	David Daney <ddaney@...iumnetworks.com>,
	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>
Subject: Re: [PATCH resend] MIPS: Allow FPU emulator to use non-stack area.

Well, I am not a subscriber to mail-list, so I read it the first time 
and some notes:

1)  David's approach would likely work for FPU emulation but unlikely 
works for MIPS Rel 2/Rel 1/ MIPS I emulation in MIPS R6 architecture. 
The reason is that the first MIPS R2 instruction (removed from MIPS R6) 
can be hit long before GLIBC/bionic/etc can determine how to use 
properly a new system call. And that instruction needs to be emulated. I 
actually hit this problem with ssh-keygen first and referred to  FPU 
emulation because I got it later, during my attempt to salvage a situation.

2)  The issue of uMIPS ADDIUPC and similar instructions are overblown in 
my opinion. Never of them are memory-related and their emulation in 
BD-slot can be easily done in kernel and that actually accelerates an 
emulation. Look at piece of code which I wrote to accelerate an 
emulation of some instructions in BD-slot of JR instruction:

         switch (MIPSInst_OPCODE(ir)) {
         case addiu_op:
                 if (MIPSInst_RT(ir))
                         regs->regs[MIPSInst_RT(ir)] =
(s32)regs->regs[MIPSInst_RS(ir)] +
                                 (s32)MIPSInst_SIMM(ir);
                 return(0);
#ifdef CONFIG_64BIT
         case daddiu_op:
                 if (MIPSInst_RT(ir))
                         regs->regs[MIPSInst_RT(ir)] =
(s64)regs->regs[MIPSInst_RS(ir)] +
(s64)MIPSInst_SIMM(ir);
return(0);
#endif

Five lines per instruction.

3)  The signal happened during execution of emulated instruction - 
signals are under control of kernel and we can easily delay a signal 
during execution of emulated instruction until return from do_dsemulret. 
It is not a big deal - nor code, nor performance. Thank you for good point.

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.

- Leonid.

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