[Qemu-devel] [PATCH 2/3] target/arm: Only flush tlb if ASID changes

[Richard Henderson <richard.henderson@linaro.org>]

Since QEMU does not implement ASIDs, changes to the ASID must flush the
tlb.  However, if the ASID does not change there is no reason to flush.

In testing a boot of the Ubuntu installer to the first menu, this reduces
the number of flushes by 30%, or nearly 600k instances.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
---
 target/arm/helper.c | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/target/arm/helper.c b/target/arm/helper.c
index 24bbde4f76..ed70ac645e 100644
--- a/target/arm/helper.c
+++ b/target/arm/helper.c
@@ -2709,12 +2709,10 @@ static void vmsa_tcr_el1_write(CPUARMState *env, const ARMCPRegInfo *ri,
 static void vmsa_ttbr_write(CPUARMState *env, const ARMCPRegInfo *ri,
                             uint64_t value)
 {
-    /* 64 bit accesses to the TTBRs can change the ASID and so we
-     * must flush the TLB.
-     */
-    if (cpreg_field_is_64bit(ri)) {
+    /* If the ASID changes (with a 64-bit write), we must flush the TLB.  */
+    if (cpreg_field_is_64bit(ri) &&
+        extract64(raw_read(env, ri) ^ value, 48, 16) != 0) {
         ARMCPU *cpu = arm_env_get_cpu(env);
-
         tlb_flush(CPU(cpu));
     }
     raw_write(env, ri, value);
-- 
2.17.2