linux-kernel - Re: [PATCH] MIPS: Fix exception entry when CONFIG

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Message-ID: <9a2d2b4b-9b6a-a7e2-78be-ff6a019d6e05@mips.com>
Date:   Wed, 15 Nov 2017 10:32:37 +0000
From:   Matt Redfearn <matt.redfearn@...s.com>
To:     "Maciej W. Rozycki" <macro@...s.com>,
        James Hogan <james.hogan@...s.com>
CC:     Corey Minyard <cminyard@...sta.com>,
        Ralf Baechle <ralf@...ux-mips.org>,
        Matthew Fortune <matthew.fortune@...s.com>,
        <linux-mips@...ux-mips.org>, <linux-kernel@...r.kernel.org>,
        "Jason A. Donenfeld" <jason@...c4.com>,
        "Paul Burton" <paul.burton@...s.com>
Subject: Re: [PATCH] MIPS: Fix exception entry when CONFIG_EVA enabled



On 13/11/17 10:47, Maciej W. Rozycki wrote:
> On Tue, 31 Oct 2017, James Hogan wrote:
> 
>>> I looked this over pretty carefully and it looks correct to me.  It
>>> makes no difference
>>> in the instructions generated by the non-EVA case.  I shouldn't have
>>> missed this :(.
>>>
>>> Reviewed-by: Corey Minyard <cminyard@...sta.com>
>>
>> Yeh, having stared at it for a little while it looks correct to me too.
>>
>> Reviewed-by: James Hogan <jhogan@...nel.org>
> 
>   How about getting rid of the `noreorder' mode and the manually scheduled
> delay slot here altogether though, as I outlined?
> 
>    Maciej
> 

Hi Maciej,

I like the change you propose, however I can't coax GAS to reorder the 
instructions appropriately. With this patch on top of 4.14:

--- a/arch/mips/include/asm/stackframe.h
+++ b/arch/mips/include/asm/stackframe.h
@@ -195,14 +195,16 @@
                 .set    push
                 .set    noat
                 .set    reorder
-               mfc0    k0, CP0_STATUS
-               sll     k0, 3           /* extract cu0 bit */
-               .set    noreorder
-               bltz    k0, 8f
-                move   k0, sp
+               mfc0    k1, CP0_STATUS
+               sll     k1, 3           /* extract cu0 bit */
+
+               move    k0, sp
                 .if \docfi
                 .cfi_register sp, k0
                 .endif
+
+               bltz    k1, 8f
+
  #ifdef CONFIG_EVA
                 /*
                  * Flush interAptiv's Return Prediction Stack (RPS) by 
writing
@@ -228,7 +230,6 @@
                 MFC0    k0, CP0_ENTRYHI
                 MTC0    k0, CP0_ENTRYHI
  #endif
-               .set    reorder
                 /* Called from user mode, new stack. */
                 get_saved_sp docfi=\docfi tosp=1
  8:


The generated assembly is:

80405d00 <handle_int>:
80405d00:       401b6000        mfc0    k1,c0_status
80405d04:       001bd8c0        sll     k1,k1,0x3
80405d08:       03a0d025        move    k0,sp
80405d0c:       07600007        bltz    k1,80405d2c <handle_int+0x2c>
80405d10:       00000000        nop
80405d14:       401a2000        mfc0    k0,c0_context

Apparently GAS has not been able to reorder the move into the branch 
delay slot for some reason. Any ideas?

Thanks,
Matt