arm64: fpsimd: avoid restoring fpcr if the contents haven't changed

Writing to the FPCR is commonly implemented as a self-synchronising
operation in the CPU, so avoid writing to the register when the saved
value matches that in the hardware already.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

1 files changed, 15 insertions, 2 deletions

diff --git a/arch/arm64/include/asm/fpsimdmacros.h b/arch/arm64/include/asm/fpsimdmacros.h
index 768414d55e64..007618b8188c 100644
--- a/arch/arm64/include/asm/fpsimdmacros.h
+++ b/arch/arm64/include/asm/fpsimdmacros.h
@@ -40,6 +40,19 @@
         str     w\tmpnr, [\state, #16 * 2 + 4]
 .endm
 
+.macro fpsimd_restore_fpcr state, tmp
+        /*
+         * Writes to fpcr may be self-synchronising, so avoid restoring
+         * the register if it hasn't changed.
+         */
+        mrs     \tmp, fpcr
+        cmp     \tmp, \state
+        b.eq    9999f
+        msr     fpcr, \state
+9999:
+.endm
+
+/* Clobbers \state */
 .macro fpsimd_restore state, tmpnr
         ldp     q0, q1, [\state, #16 * 0]
         ldp     q2, q3, [\state, #16 * 2]
@@ -60,7 +73,7 @@
         ldr     w\tmpnr, [\state, #16 * 2]
         msr     fpsr, x\tmpnr
         ldr     w\tmpnr, [\state, #16 * 2 + 4]
-        msr     fpcr, x\tmpnr
+        fpsimd_restore_fpcr x\tmpnr, \state
 .endm
 
 .altmacro
@@ -84,7 +97,7 @@
 .macro fpsimd_restore_partial state, tmpnr1, tmpnr2
         ldp     w\tmpnr1, w\tmpnr2, [\state]
         msr     fpsr, x\tmpnr1
-        msr     fpcr, x\tmpnr2
+        fpsimd_restore_fpcr x\tmpnr2, x\tmpnr1
         adr     x\tmpnr1, 0f
         ldr     w\tmpnr2, [\state, #8]
         add     \state, \state, x\tmpnr2, lsl #4