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[PATCH] Close tiny holes in the SMTC IPI replay system. Signed-off-by: K

Subject: [PATCH] Close tiny holes in the SMTC IPI replay system. Signed-off-by: Kevin D. Kissell <kevink@paralogos.com>
From: Kevin D. Kissell <kevink@paralogos.com>
Date: Tue, 9 Sep 2008 21:35:01 +0200
---
 arch/mips/kernel/entry.S      |   10 +++---
 include/asm-mips/stackframe.h |   72 ++++++++++++++++++++++++++++++++++------
 2 files changed, 66 insertions(+), 16 deletions(-)

diff --git a/arch/mips/kernel/entry.S b/arch/mips/kernel/entry.S
index e29598a..ffa3310 100644
--- a/arch/mips/kernel/entry.S
+++ b/arch/mips/kernel/entry.S
@@ -79,11 +79,6 @@ FEXPORT(syscall_exit)
 
 FEXPORT(restore_all)                   # restore full frame
 #ifdef CONFIG_MIPS_MT_SMTC
-/* Detect and execute deferred IPI "interrupts" */
-       LONG_L  s0, TI_REGS($28)
-       LONG_S  sp, TI_REGS($28)
-       jal     deferred_smtc_ipi
-       LONG_S  s0, TI_REGS($28)
 #ifdef CONFIG_MIPS_MT_SMTC_IM_BACKSTOP
 /* Re-arm any temporarily masked interrupts not explicitly "acked" */
        mfc0    v0, CP0_TCSTATUS
@@ -112,6 +107,11 @@ FEXPORT(restore_all)                       # restore full 
frame
        xor     t0, t0, t3
        mtc0    t0, CP0_TCCONTEXT
 #endif /* CONFIG_MIPS_MT_SMTC_IM_BACKSTOP */
+/* Detect and execute deferred IPI "interrupts" */
+       LONG_L  s0, TI_REGS($28)
+       LONG_S  sp, TI_REGS($28)
+       jal     deferred_smtc_ipi
+       LONG_S  s0, TI_REGS($28)
 #endif /* CONFIG_MIPS_MT_SMTC */
        .set    noat
        RESTORE_TEMP
diff --git a/include/asm-mips/stackframe.h b/include/asm-mips/stackframe.h
index 051e1af..4c37c4e 100644
--- a/include/asm-mips/stackframe.h
+++ b/include/asm-mips/stackframe.h
@@ -297,14 +297,31 @@
 #ifdef CONFIG_MIPS_MT_SMTC
                .set    mips32r2
                /*
-                * This may not really be necessary if ints are already
-                * inhibited here.
+                * We need to make sure the read-modify-write
+                * of Status below isn't perturbed by an interrupt
+                * or cross-TC access, so we need to do at least a DMT,
+                * protected by an interrupt-inhibit. But setting IXMT
+                * also creates a few-cycle window where an IPI could
+                * be queued and not be detected before potentially
+                * returning to a WAIT or user-mode loop. It must be
+                * replayed.
+                *
+                * We're in the middle of a context switch, and
+                * we can't dispatch it directly without trashing
+                * some registers, so we'll try to detect this unlikely
+                * case and program a software interrupt in the VPE,
+                * as would be done for a cross-VPE IPI.  To accomodate
+                * the handling of that case, we're doing a DVPE instead
+                * of just a DMT here to protect against other threads.
+                * This is a lot of cruft to cover a tiny window.
+                * If you can find a better design, implement it!
+                *
                 */
                mfc0    v0, CP0_TCSTATUS
                ori     v0, TCSTATUS_IXMT
                mtc0    v0, CP0_TCSTATUS
                _ehb
-               DMT     5                               # dmt a1
+               DVPE    5                               # dvpe a1
                jal     mips_ihb
 #endif /* CONFIG_MIPS_MT_SMTC */
                mfc0    a0, CP0_STATUS
@@ -325,17 +342,50 @@
  */
                LONG_L  v1, PT_TCSTATUS(sp)
                _ehb
-               mfc0    v0, CP0_TCSTATUS
+               mfc0    a0, CP0_TCSTATUS
                andi    v1, TCSTATUS_IXMT
-               /* We know that TCStatua.IXMT should be set from above */
-               xori    v0, v0, TCSTATUS_IXMT
-               or      v0, v0, v1
-               mtc0    v0, CP0_TCSTATUS
-               _ehb
-               andi    a1, a1, VPECONTROL_TE
+               bnez    v1, 0f
+
+/*
+ * We'd like to detect any IPIs queued in the tiny window
+ * above and request an software interrupt to service them
+ * when we ERET.
+ *
+ * Computing the offset into the IPIQ array of the executing
+ * TC's IPI queue in-line would be tedious.  We use part of
+ * the TCContext register to hold 16 bits of offset that we
+ * can add in-line to find the queue head.
+ */
+               mfc0    v0, CP0_TCCONTEXT
+               la      a2, IPIQ
+               srl     v0, v0, 16
+               addu    a2, a2, v0
+               LONG_L  v0, 0(a2)
+               beqz    v0, 0f
+/*
+ * If we have a queue, provoke dispatch within the VPE by setting C_SW1
+ */
+               mfc0    v0, CP0_CAUSE
+               ori     v0, v0, C_SW1
+               mtc0    v0, CP0_CAUSE
+0:
+               /*
+                * This test should really never branch but
+                * let's be prudent here.  Having atomized
+                * the shared register modifications, we can
+                * now EVPE, and must do so before interrupts
+                * are potentially re-enabled.
+                */
+               andi    a1, a1, MVPCONTROL_EVP
                beqz    a1, 1f
-               emt
+               evpe
 1:
+               /* We know that TCStatua.IXMT should be set from above */
+               xori    a0, a0, TCSTATUS_IXMT
+               or      a0, a0, v1
+               mtc0    a0, CP0_TCSTATUS
+               _ehb
+
                .set    mips0
 #endif /* CONFIG_MIPS_MT_SMTC */
                LONG_L  v1, PT_EPC(sp)
-- 
1.5.3.3


--------------080009040507040709070608--

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