[MIPS] SMTC: Interrupt mask backstop hack

To support multiple TC microthreads acting as "CPUs" within a VPE,
VPE-wide interrupt mask bits must be specially manipulated during
interrupt handling. To support legacy drivers and interrupt controller
management code, SMTC has a "backstop" to track and if necessary restore
the interrupt mask. This has some performance impact on interrupt service
overhead. Disable it only if you know what you are doing.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

3 files changed, 17 insertions, 0 deletions

diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
index a00fabe2e4e0..49f02e351244 100644
--- a/arch/mips/Kconfig
+++ b/arch/mips/Kconfig
@@ -1404,6 +1404,19 @@ config MIPS_MT_SMTC_INSTANT_REPLAY
           it off), but ensures that IPIs are handled promptly even under
           heavy I/O interrupt load.
 
+config MIPS_MT_SMTC_IM_BACKSTOP
+        bool "Use per-TC register bits as backstop for inhibited IM bits"
+        depends on MIPS_MT_SMTC
+        default y
+        help
+          To support multiple TC microthreads acting as "CPUs" within
+          a VPE, VPE-wide interrupt mask bits must be specially manipulated
+          during interrupt handling. To support legacy drivers and interrupt
+          controller management code, SMTC has a "backstop" to track and
+          if necessary restore the interrupt mask. This has some performance
+          impact on interrupt service overhead. Disable it only if you know
+          what you are doing.
+
 config MIPS_VPE_LOADER_TOM
         bool "Load VPE program into memory hidden from linux"
         depends on MIPS_VPE_LOADER
diff --git a/arch/mips/kernel/entry.S b/arch/mips/kernel/entry.S
index 686249c5c328..e29598ae939d 100644
--- a/arch/mips/kernel/entry.S
+++ b/arch/mips/kernel/entry.S
@@ -84,6 +84,7 @@ FEXPORT(restore_all)			# restore full frame
         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
         ori     v1, v0, TCSTATUS_IXMT
@@ -110,6 +111,7 @@ FEXPORT(restore_all)			# restore full frame
         _ehb
         xor     t0, t0, t3
         mtc0    t0, CP0_TCCONTEXT
+#endif /* CONFIG_MIPS_MT_SMTC_IM_BACKSTOP */
 #endif /* CONFIG_MIPS_MT_SMTC */
         .set    noat
         RESTORE_TEMP
diff --git a/arch/mips/kernel/genex.S b/arch/mips/kernel/genex.S
index 297bd56c2347..c0f19d638b98 100644
--- a/arch/mips/kernel/genex.S
+++ b/arch/mips/kernel/genex.S
@@ -243,9 +243,11 @@ NESTED(except_vec_vi_handler, 0, sp)
          */
         mfc0    t1, CP0_STATUS
         and     t0, a0, t1
+#ifdef CONFIG_MIPS_MT_SMTC_IM_BACKSTOP
         mfc0    t2, CP0_TCCONTEXT
         or      t0, t0, t2
         mtc0    t0, CP0_TCCONTEXT
+#endif /* CONFIG_MIPS_MT_SMTC_IM_BACKSTOP */
         xor     t1, t1, t0
         mtc0    t1, CP0_STATUS
         _ehb