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Re: [Qemu-devel] [PATCH v2 1/5] backdoor: Add documentation

From: Anthony Liguori
Subject: Re: [Qemu-devel] [PATCH v2 1/5] backdoor: Add documentation
Date: Tue, 06 Dec 2011 16:50:19 -0600
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20110831 Lightning/1.0b2 Thunderbird/3.1.13

On 12/05/2011 04:22 PM, Lluís Vilanova wrote:
Signed-off-by: Lluís Vilanova<address@hidden>
  docs/backdoor.txt |  167 +++++++++++++++++++++++++++++++++++++++++++++++++++++
  1 files changed, 167 insertions(+), 0 deletions(-)
  create mode 100644 docs/backdoor.txt

diff --git a/docs/backdoor.txt b/docs/backdoor.txt
new file mode 100644
index 0000000..1c6502a
--- /dev/null
+++ b/docs/backdoor.txt
@@ -0,0 +1,167 @@
+= Backdoor communication channel =
+== Introduction ==
+This document describes how the guest can use the backdoor communication 
+to interact with user-provided code inside QEMU.
+The backdoor provides a lightweight and guest-initiated communication channel
+between code running inside the guest system and code in QEMU, including both
+QEMU in 'softmmu' and 'user' modes.
+The semantics of the backdoor channel are up to the user, who must provide the
+implementation of the QEMU-side callbacks used when the backdoor channel is
+On the guest side, code can be linked against a simple library provided in QEMU
+to interface with the backdoor channel.
+The features of this mechanism are:
+* Minimal setup for the guest.
+* Independent of guest architecture.
+* Works with 'softmmu' and 'user' mode.
+* Negligible guest overhead; guest invocations of the backdoor channel does not
+  go through any OS abstraction, except during the setup of the communication
+  channel.
+* Negligible host overhead; invocations of the backdoor channel are interpreted
+  by QEMU, while a side-channel can be used as regular memory to communicate
+  bulk data without any extra overhead.
+* The user-provided backdoor callbacks can perform arbitrary actions on the
+  guest system (e.g., read or write memory, change register values, etc.).
+== QEMU-side code ==
+1. Create the "Makefile" to build the user-provided backdoor channel library:
+    mkdir /tmp/my-backdoor-qemu
+    cat>  /tmp/my-backdoor-qemu/Makefile<<EOF
+    include $(BUILD_DIR)/config-host.mak
+    include $(BUILD_DIR)/$(TARGET_DIR)../config-target.mak
+    include $(SRC_PATH)/rules.mak
+    vpath %.c /tmp/my-backdoor-qemu
+    libbackdoor.a: backdoor.o
+    # Include automatically generated dependency files
+    -include $(wildcard *.d)
+    EOF
+2. Implement the callbacks declared in "backdoor/qemu/qemu-backdoor.h":
+    cat>  /tmp/my-backdoor-qemu/backdoor.c<<EOF

Oh, I didn't see this.

Major Nack on this. This is not reasonable at all to do. We're not sneaking in an ad-hoc plugin interface here.


Anthony Liguori

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