Add mmio support info in docs/specs/pvpanic.txt.

Signed-off-by: Peng Hao <peng.h...@zte.com.cn>
---
 docs/specs/pvpanic.txt | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/docs/specs/pvpanic.txt b/docs/specs/pvpanic.txt
index c7bbacc..5d8e9dc 100644
--- a/docs/specs/pvpanic.txt
+++ b/docs/specs/pvpanic.txt
@@ -1,7 +1,7 @@
 PVPANIC DEVICE
 ==============
 
-pvpanic device is a simulated ISA device, through which a guest panic
+pvpanic device is a simulated device, through which a guest panic
 event is sent to qemu, and a QMP event is generated. This allows
 management apps (e.g. libvirt) to be notified and respond to the event.
 
@@ -9,6 +9,10 @@ The management app has the option of waiting for 
GUEST_PANICKED events,
 and/or polling for guest-panicked RunState, to learn when the pvpanic
 device has fired a panic event.
 
+When pvpanic device is implemented as a ISA device, it supports IOPORT 
+mode. If pvpanic device supports MMIO mode, it will be implemented as
+a SYSBUS device.
+
 ISA Interface
 -------------
 
@@ -19,6 +23,13 @@ Software should set only bits both itself and the device 
recognize.
 Currently, only bit 0 is recognized, setting it indicates a guest panic
 has happened.
 
+SYSBUS Interface
+--------------
+
+It is basically the same as ISA interface except that it uses MMIO. Pvpanic 
exposes
+a address space region 0x09060000--0x09060001 in arm virt machine.
+Currently only the first byte is used. 
+
 ACPI Interface
 --------------
 
-- 
1.8.3.1


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