Re: [PATCH] MIPS: R6: emulation of PC-relative instructions

From: Leonid Yegoshin
Date: Tue Aug 11 2015 - 14:24:16 EST

On 08/11/2015 07:41 AM, Markos Chandras wrote:

On Wed, Aug 05, 2015 at 04:53:43PM -0700, Leonid Yegoshin wrote:
if (nir) {
err = mipsr6_emul(regs, nir);
if (err > 0) {
+ regs->cp0_epc = nepc;
Does this change belog to this patch? If so why?

Yes, it is needed for correct address calculation. Until PC-relative instruction emulation it was not needed in dsemul().

Maybe a comment would help?
It does feel like it fixes a different problem but I haven't read your patch in depth.

#include "ieee754.h"
Can we simply avoid the if/def for R6 please? Just leave this function as is and
use if(cpu_has_mips_r6) when calling it. If you can't do that, please explain
Yes, we can. But we have a bunch of that in many places and somewhere it is difficult to avoid "#ifdef".
I just like to have an uniform standard.

Besides that "#ifdef" assures quickly that it is a build time choice but not runtime.

+static int mipsr6_pc(struct pt_regs *regs, mips_instruction inst, unsigned long cpc,
+ unsigned long bpc, unsigned long r31)
+ union mips_instruction ir = (union mips_instruction)inst;
+ register unsigned long vaddr;
+ unsigned int val;
+ int err = SIGILL;
+ if (ir.rel_format.opcode != pcrel_op)
+ return SIGILL;
+ switch (ir.rel_format.op) {
+ case addiupc_op:
+ vaddr = regs->cp0_epc + (ir.rel_format.simmediate << 2);
+ if (config_enabled(CONFIG_64BIT) && !(regs->cp0_status & ST0_UX))
+ __asm__ __volatile__("sll %0, %0, 0":"+&r"(vaddr)::);
+ regs->regs[] = vaddr;
+ return 0;
Could you use cpu_has_mips64 and avoid the if/def

No we can't - cpu_has_mips64 is a CPU ISA feature but here is a kernel code capability restriction. The difference happens during running MIPS32 kernel on MIPS64 processor. We should not emulate MIPS64 instructions on MIPS32 kernel.

and return SIGILL if it is not

The common return standard for emulation functions in MIPS is:

0 - OK, emulation done
SIGILL - doesn't recognize an instruction, it still may be some another way to fix a problem or SIGILL.
other - some trouble or whatever (non-standardized, in MIPS R2 emulator it has subcodes)

I don't see a reason to change it and have here a special standard.

- Leonid.

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