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Re: [PATCH 2/5] MIPS: Add instrunction format for BREAK and SYSCALL

To: "Maciej W. Rozycki" <macro@linux-mips.org>
Subject: Re: [PATCH 2/5] MIPS: Add instrunction format for BREAK and SYSCALL
From: David Daney <ddaney@caviumnetworks.com>
Date: Tue, 03 Aug 2010 13:59:43 -0700
Cc: linux-mips@linux-mips.org, ralf@linux-mips.org, ananth@in.ibm.com, anil.s.keshavamurthy@intel.com, davem@davemloft.net, masami.hiramatsu.pt@hitachi.com, linux-kernel@vger.kernel.org, hschauhan@nulltrace.org
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On 08/03/2010 01:54 PM, Maciej W. Rozycki wrote:
On Tue, 3 Aug 2010, David Daney wrote:

diff --git a/arch/mips/include/asm/inst.h b/arch/mips/include/asm/inst.h
index 6489f00..444ff71 100644
--- a/arch/mips/include/asm/inst.h
+++ b/arch/mips/include/asm/inst.h
@@ -247,6 +247,12 @@ struct ma_format { /* FPU multipy and add format (MIPS IV) 
*/
        unsigned int fmt : 2;
  };

+struct b_format { /* BREAK and SYSCALL */
+       unsigned int opcode:6;
+       unsigned int code:20;
+       unsigned int func:6;
+};
+
  #elif defined(__MIPSEL__)

  struct j_format {     /* Jump format */

  Please note the code field of the BREAK instruction is by toolchain
convention (bug-compatibility with the original MIPS assembler or
suchlike) treated as a pair of swapped 10-bit fields -- you may want to
double-check consistency of interpretation with usage elsewhere.


Indeed, I am familiar with that fact.  From patch 3/5 we have:

.
.
.
+void __kprobes jprobe_return(void)
+{
+       /* Assembler quirk necessitates this '0,code' business.  */
+       asm volatile(
+               "break 0,%0\n\t"
+               ".globl jprobe_return_end\n"
+               "jprobe_return_end:\n"
+               : : "n" (BRK_KPROBE_BP) : "memory");
+}
.
.
.

The 'break 0,code' construct causes gas to emit values that are compatible with the other use of struct b_format in the patch set.

David Daney

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