[RFC PATCH] target/arm: add RAZ/WI handling for DBGDTR[TX|RX]

["Alex Bennée" <alex.bennee@linaro.org>]

The commit b3aa2f2128 (target/arm: provide stubs for more external
debug registers) was added to handle HyperV's unconditional usage of
Debug Communications Channel. It turns out that Linux will similarly
break if you enable CONFIG_HVC_DCC "ARM JTAG DCC console".

Extend the registers we RAZ/WI set to avoid this.

Cc: Anders Roxell <anders.roxell@linaro.org>
Cc: Evgeny Iakovlev <eiakovlev@linux.microsoft.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
---
 target/arm/debug_helper.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/target/arm/debug_helper.c b/target/arm/debug_helper.c
index dfc8b2a1a5..d41cc643b1 100644
--- a/target/arm/debug_helper.c
+++ b/target/arm/debug_helper.c
@@ -949,8 +949,10 @@ static const ARMCPRegInfo debug_cp_reginfo[] = {
       .access = PL0_R, .accessfn = access_tdcc,
       .type = ARM_CP_CONST, .resetvalue = 0 },
     /*
-     * OSDTRRX_EL1/OSDTRTX_EL1 are used for save and restore of DBGDTRRX_EL0.
-     * It is a component of the Debug Communications Channel, which is not implemented.
+     * These registers belong to the Debug Communications Channel,
+     * which is not implemented. However we implement RAZ/WI behaviour
+     * with trapping to prevent spurious SIGILLs if the guest OS does
+     * access them as the support cannot be probed for.
      */
     { .name = "OSDTRRX_EL1", .state = ARM_CP_STATE_BOTH, .cp = 14,
       .opc0 = 2, .opc1 = 0, .crn = 0, .crm = 0, .opc2 = 2,
@@ -960,6 +962,11 @@ static const ARMCPRegInfo debug_cp_reginfo[] = {
       .opc0 = 2, .opc1 = 0, .crn = 0, .crm = 3, .opc2 = 2,
       .access = PL1_RW, .accessfn = access_tdcc,
       .type = ARM_CP_CONST, .resetvalue = 0 },
+    /* DBGDTRTX_EL0/DBGDTRRX_EL0 depend on direction */
+    { .name = "DBGDTR_EL0", .state = ARM_CP_STATE_BOTH, .cp = 14,
+      .opc0 = 2, .opc1 = 3, .crn = 0, .crm = 5, .opc2 = 0,
+      .access = PL0_RW, .accessfn = access_tdcc,
+      .type = ARM_CP_CONST, .resetvalue = 0 },
     /*
      * OSECCR_EL1 provides a mechanism for an operating system
      * to access the contents of EDECCR. EDECCR is not implemented though,
-- 
2.39.2