[v1] docs: Add to gdbstub documentation the PhyMemMode

[Jon Doron <arilou@gmail.com>]

The PhyMemMode gdb extension command was missing from the gdb.rst
document.

Signed-off-by: Jon Doron <arilou@gmail.com>
---
 docs/system/gdb.rst | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/docs/system/gdb.rst b/docs/system/gdb.rst
index a40145fcf8..abda961e2b 100644
--- a/docs/system/gdb.rst
+++ b/docs/system/gdb.rst
@@ -87,3 +87,23 @@ three commands you can query and set the single step behavior:
       (gdb) maintenance packet Qqemu.sstep=0x5
       sending: "qemu.sstep=0x5"
       received: "OK"
+
+
+Another feature that QEMU gdbstub provides is to toggle the memory GDB
+works with, by default GDB will show the current process memory respecting
+the virtual address translation.
+
+If you want to examine/change the physical memory you can set the gdbstub
+to work with the physical memory rather with the virtual one.
+
+The memory mode can be checked by sending the following command:
+
+``maintenance packet qqemu.PhyMemMode``
+    This will return either 0 or 1, 1 indicates you are currently in the
+    physical memory mode.
+
+``maintenance packet Qqemu.PhyMemMode:1``
+    This will change the memory mode to physical memory.
+
+``maintenance packet Qqemu.PhyMemMode:0``
+    This will change it back to normal memory mode.
-- 
2.25.1