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[PATCH 2/2] MIPS: Don't branch to eret in TLB refill.

To: linux-mips@linux-mips.org, ralf@linux-mips.org
Subject: [PATCH 2/2] MIPS: Don't branch to eret in TLB refill.
From: David Daney <ddaney@caviumnetworks.com>
Date: Wed, 13 May 2009 13:48:37 -0700
Cc: David Daney <ddaney@caviumnetworks.com>
In-reply-to: <4A0B31C6.9050804@caviumnetworks.com>
Original-recipient: rfc822;linux-mips@linux-mips.org
References: <4A0B31C6.9050804@caviumnetworks.com>
Sender: linux-mips-bounce@linux-mips.org
If the TLB refill handler is too big and needs to be split, there is
no need to branch around the split if the branch target would be an
eret.  Since the eret returns from the handler, control flow never
passes it.  A branch to an eret is equivalent to the eret itself.

Signed-off-by: David Daney <ddaney@caviumnetworks.com>
---
 arch/mips/mm/tlbex.c |   66 ++++++++++++++++++++++++++++++-------------------
 1 files changed, 40 insertions(+), 26 deletions(-)

diff --git a/arch/mips/mm/tlbex.c b/arch/mips/mm/tlbex.c
index d99ed78..0a88383 100644
--- a/arch/mips/mm/tlbex.c
+++ b/arch/mips/mm/tlbex.c
@@ -664,6 +664,7 @@ static void __cpuinit build_r4000_tlb_refill_handler(void)
        struct uasm_reloc *r = relocs;
        u32 *f;
        unsigned int final_len;
+       int split_on_eret;
 
        memset(tlb_handler, 0, sizeof(tlb_handler));
        memset(labels, 0, sizeof(labels));
@@ -693,6 +694,12 @@ static void __cpuinit build_r4000_tlb_refill_handler(void)
        build_tlb_write_entry(&p, &l, &r, tlb_random);
        uasm_l_leave(&l, p);
        uasm_i_eret(&p); /* return from trap */
+       /*
+        * Check to see if the eret was the last instruction before
+        * the split.  If it was, there will be no need to branch
+        * around the split, because we are returning.
+        */
+       split_on_eret = (p - tlb_handler == MIPS64_REFILL_INSNS);
 
 #ifdef CONFIG_64BIT
        build_get_pgd_vmalloc64(&p, &l, &r, K0, K1);
@@ -732,36 +739,43 @@ static void __cpuinit build_r4000_tlb_refill_handler(void)
                uasm_copy_handler(relocs, labels, tlb_handler, p, f);
                final_len = p - tlb_handler;
        } else {
-               /*
-                * Split two instructions before the end.  One for the
-                * branch and one for the instruction in the delay
-                * slot.
-                */
-               u32 *split = tlb_handler + MIPS64_REFILL_INSNS - 2;
-
-               /*
-                * Find the split point.  If the branch would fall in
-                * a delay slot, we must back up an additional
-                * instruction so that it is no longer in a delay
-                * slot.
-                */
-               if (uasm_insn_has_bdelay(relocs, split - 1))
-                       split--;
-
+               u32 *split;
+               if (split_on_eret) {
+                       split = tlb_handler + MIPS64_REFILL_INSNS;
+               } else {
+                       /*
+                        * Split two instructions before the end.  One
+                        * for the branch and one for the instruction
+                        * in the delay slot.
+                        */
+                       u32 *split = tlb_handler + MIPS64_REFILL_INSNS - 2;
+
+                       /*
+                        * Find the split point.  If the branch would
+                        * fall in a delay slot, we must back up an
+                        * additional instruction so that it is no
+                        * longer in a delay slot.
+                        */
+                       if (uasm_insn_has_bdelay(relocs, split - 1))
+                               split--;
+               }
                /* Copy first part of the handler. */
                uasm_copy_handler(relocs, labels, tlb_handler, split, f);
                f += split - tlb_handler;
 
-               /* Insert branch. */
-               uasm_l_split(&l, final_handler);
-               uasm_il_b(&f, &r, label_split);
-               if (uasm_insn_has_bdelay(relocs, split))
-                       uasm_i_nop(&f);
-               else {
-                       uasm_copy_handler(relocs, labels, split, split + 1, f);
-                       uasm_move_labels(labels, f, f + 1, -1);
-                       f++;
-                       split++;
+               if (!split_on_eret) {
+                       /* Insert branch. */
+                       uasm_l_split(&l, final_handler);
+                       uasm_il_b(&f, &r, label_split);
+                       if (uasm_insn_has_bdelay(relocs, split))
+                               uasm_i_nop(&f);
+                       else {
+                               uasm_copy_handler(relocs, labels,
+                                                 split, split + 1, f);
+                               uasm_move_labels(labels, f, f + 1, -1);
+                               f++;
+                               split++;
+                       }
                }
 
                /* Copy the rest of the handler. */
-- 
1.6.0.6


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