target/arm: Enable FEAT_DGH for -cpu max

This extension concerns not merging memory access, which TCG does
not implement.  Thus we can trivially enable this feature.
Add a comment to handle_hint for the DGH instruction, but no code.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20220506180242.216785-23-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

docs/system/arm/emulation.rst

@@ -17,6 +17,7 @@ the following architecture extensions:
 - FEAT_CSV2_1p2 (Cache speculation variant 2, version 1.2)
 - FEAT_CSV2_2 (Cache speculation variant 2, version 2)
 - FEAT_CSV3 (Cache speculation variant 3)
+- FEAT_DGH (Data gathering hint)
 - FEAT_DIT (Data Independent Timing instructions)
 - FEAT_DPB (DC CVAP instruction)
 - FEAT_Debugv8p2 (Debug changes for v8.2)

target/arm/cpu64.c

@@ -738,6 +738,7 @@ static void aarch64_max_initfn(Object *obj)
     t = FIELD_DP64(t, ID_AA64ISAR1, SB, 1);       /* FEAT_SB */
     t = FIELD_DP64(t, ID_AA64ISAR1, SPECRES, 1);  /* FEAT_SPECRES */
     t = FIELD_DP64(t, ID_AA64ISAR1, BF16, 1);     /* FEAT_BF16 */
+    t = FIELD_DP64(t, ID_AA64ISAR1, DGH, 1);      /* FEAT_DGH */
     t = FIELD_DP64(t, ID_AA64ISAR1, I8MM, 1);     /* FEAT_I8MM */
     cpu->isar.id_aa64isar1 = t;
 

target/arm/translate-a64.c

@@ -1427,6 +1427,7 @@ static void handle_hint(DisasContext *s, uint32_t insn,
         break;
     case 0b00100: /* SEV */
     case 0b00101: /* SEVL */
+    case 0b00110: /* DGH */
         /* we treat all as NOP at least for now */
         break;
     case 0b00111: /* XPACLRI */