target-mips: add Unified Hosting Interface (UHI) support

Add UHI semihosting support for MIPS. QEMU run with "-semihosting" option
will alter the behaviour of SDBBP 1 instruction -- UHI operation will be
called instead of generating a debug exception.

Also tweak Malta's pseudo-bootloader. On CPU reset the $4 register is set
to -1 if semihosting arguments are passed to indicate that the UHI
operations should be used to obtain input arguments.

Signed-off-by: Leon Alrae <leon.alrae@imgtec.com>
Reviewed-by: Aurelien Jarno <aurelien@aurel32.net>

1 files changed, 8 insertions, 1 deletions

diff --git a/hw/mips/mips_malta.c b/hw/mips/mips_malta.c
index a5d64d5030..3082e75340 100644
--- a/hw/mips/mips_malta.c
+++ b/hw/mips/mips_malta.c
@@ -53,6 +53,7 @@
 #include "qemu/error-report.h"
 #include "hw/empty_slot.h"
 #include "sysemu/kvm.h"
+#include "exec/semihost.h"
 
 //#define DEBUG_BOARD_INIT
 
@@ -634,7 +635,13 @@ static void write_bootloader (CPUMIPSState *env, uint8_t *base,
 
     /* Second part of the bootloader */
     p = (uint32_t *) (base + 0x580);
-    stl_p(p++, 0x24040002);                                      /* addiu a0, zero, 2 */
+
+    if (semihosting_get_argc()) {
+        /* Preserve a0 content as arguments have been passed */
+        stl_p(p++, 0x00000000);                         /* nop */
+    } else {
+        stl_p(p++, 0x24040002);                         /* addiu a0, zero, 2 */
+    }
     stl_p(p++, 0x3c1d0000 | (((ENVP_ADDR - 64) >> 16) & 0xffff)); /* lui sp, high(ENVP_ADDR) */
     stl_p(p++, 0x37bd0000 | ((ENVP_ADDR - 64) & 0xffff));        /* ori sp, sp, low(ENVP_ADDR) */
     stl_p(p++, 0x3c050000 | ((ENVP_ADDR >> 16) & 0xffff));       /* lui a1, high(ENVP_ADDR) */