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Re: [PATCH RFC v2 41/70] MIPS: mm: tlbex: Use cpu_has_mips_r2_exec_hazar

To: "Maciej W. Rozycki" <macro@linux-mips.org>
Subject: Re: [PATCH RFC v2 41/70] MIPS: mm: tlbex: Use cpu_has_mips_r2_exec_hazard for the EHB instruction
From: David Daney <ddaney.cavm@gmail.com>
Date: Mon, 23 Feb 2015 16:53:37 -0800
Cc: Markos Chandras <markos.chandras@imgtec.com>, Ralf Baechle <ralf@linux-mips.org>, linux-mips@linux-mips.org, Leonid Yegoshin <Leonid.Yegoshin@imgtec.com>
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On 02/23/2015 04:33 PM, Maciej W. Rozycki wrote:
On Mon, 23 Feb 2015, David Daney wrote:

For the version of this patch currently in mips-for-linux-next: NACK

There are two problems:

1) It breaks OCTEON, which will now crash in early boot with:

   Kernel panic - not syncing: No TLB refill handler yet (CPU type: 80)

2) The logic is broken.

The meaning of cpu_has_mips_r2_exec_hazard is that the EHB instruction is
required.  You change the meaning to be that EHB is part of the ISA.

  Well, the macro is nowhere used I'm afraid, its last use was dropped with
625c0a21, so it's rather difficult to assume any meaning to the macro.

  Also the intended meaning is clear from the commit message of 41f0e4d0,
where the macro comes from, however unfortunately not from the definition
of the macro itself.  It's a pity that along your change you did not
include an explanatory note in arch/mips/include/asm/cpu-features.h.

  Finally, I think the change made to `build_tlb_write_entry' with 625c0a21
may need to be reconsidered, as may perhaps the name itself of
`cpu_has_mips_r2_exec_hazard' (why is it this place only that the macro
was used? -- would it be better called `cpu_has_tlbw_exec_hazard'
instead?), and then we'll need `cpu_has_ehb' or suchlike across all the
other places.


I agree with all the points you make.

The parts of tlbex.c dealing with EHB are an inconsistent mess. Since it is one of my favorite playgrounds I am probably as guilty as anyone.

The idea behind `cpu_has_mips_r2_exec_hazard' (although a better name could have been chosen, as you point out) was to get rid of the case statements based on CPU model in tlbex.c, and move the logic into the individual cpu-feature files. I thought this would make it easier to add new CPUs in the future, but it turns out that now we have a random assortment of both :-(.

David Daney


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