[PATCH v3 04/16] hw/i386/vmport: Set EAX to -1 on failed and unsupported commands

[Liran Alon]

This is used as a signal for VMware Tools to know if a command it
attempted to invoke, failed or is unsupported. As a result, VMware Tools
will either report failure to user or fallback to another backdoor command
in attempt to perform some operation.

A few examples:
* open-vm-tools TimeSyncReadHost() function fallbacks to
CMD_GETTIMEFULL command when CMD_GETTIMEFULL_WITH_LAG
fails/unsupported.
* open-vm-tools Hostinfo_NestingSupported() function verifies
EAX != -1 to check for success.
* open-vm-tools Hostinfo_VCPUInfoBackdoor() functions checks
if reserved-bit is set to indicate command is unimplemented.

Reviewed-by: Nikita Leshenko <address@hidden>
Signed-off-by: Liran Alon <address@hidden>
---
 hw/core/machine.c |  1 +
 hw/i386/vmport.c  | 19 +++++++++++++++----
 2 files changed, 16 insertions(+), 4 deletions(-)

diff --git a/hw/core/machine.c b/hw/core/machine.c
index 0496112b741c..2595a13e5650 100644
--- a/hw/core/machine.c
+++ b/hw/core/machine.c
@@ -40,6 +40,7 @@ GlobalProperty hw_compat_4_2[] = {
     { "qxl", "revision", "4" },
     { "qxl-vga", "revision", "4" },
     { "vmport", "x-read-set-eax", "off" },
+    { "vmport", "x-signal-unsupported-cmd", "off" },
 };
 const size_t hw_compat_4_2_len = G_N_ELEMENTS(hw_compat_4_2);
 
diff --git a/hw/i386/vmport.c b/hw/i386/vmport.c
index 72f09ef5cba3..e67c7bb2afea 100644
--- a/hw/i386/vmport.c
+++ b/hw/i386/vmport.c
@@ -44,9 +44,12 @@
 #define VMPORT_MAGIC   0x564D5868
 
 /* Compatibility flags for migration */
-#define VMPORT_COMPAT_READ_SET_EAX_BIT      0
-#define VMPORT_COMPAT_READ_SET_EAX          \
+#define VMPORT_COMPAT_READ_SET_EAX_BIT              0
+#define VMPORT_COMPAT_SIGNAL_UNSUPPORTED_CMD_BIT    1
+#define VMPORT_COMPAT_READ_SET_EAX              \
     (1 << VMPORT_COMPAT_READ_SET_EAX_BIT)
+#define VMPORT_COMPAT_SIGNAL_UNSUPPORTED_CMD    \
+    (1 << VMPORT_COMPAT_SIGNAL_UNSUPPORTED_CMD_BIT)
 
 #define VMPORT(obj) OBJECT_CHECK(VMPortState, (obj), TYPE_VMPORT)
 
@@ -87,17 +90,23 @@ static uint64_t vmport_ioport_read(void *opaque, hwaddr 
addr,
 
     eax = env->regs[R_EAX];
     if (eax != VMPORT_MAGIC) {
-        goto out;
+        goto err;
     }
 
     command = env->regs[R_ECX];
     trace_vmport_command(command);
     if (command >= VMPORT_ENTRIES || !s->func[command]) {
         qemu_log_mask(LOG_UNIMP, "vmport: unknown command %x\n", command);
-        goto out;
+        goto err;
     }
 
     eax = s->func[command](s->opaque[command], addr);
+    goto out;
+
+err:
+    if (s->compat_flags & VMPORT_COMPAT_SIGNAL_UNSUPPORTED_CMD) {
+        eax = UINT32_MAX;
+    }
 
 out:
     /*
@@ -189,6 +198,8 @@ static Property vmport_properties[] = {
     /* Used to enforce compatibility for migration */
     DEFINE_PROP_BIT("x-read-set-eax", VMPortState, compat_flags,
                     VMPORT_COMPAT_READ_SET_EAX_BIT, true),
+    DEFINE_PROP_BIT("x-signal-unsupported-cmd", VMPortState, compat_flags,
+                    VMPORT_COMPAT_SIGNAL_UNSUPPORTED_CMD_BIT, true),
     DEFINE_PROP_END_OF_LIST(),
 };
 
-- 
2.20.1