Signed-off-by: Lluís Vilanova <vilan...@ac.upc.edu>
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+= Backdoor communication channel =
+
+== Introduction ==
+
+This document describes how the guest can use the backdoor communication
channel
+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
+invoked.
+
+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
+ #include "backdoor/qemu/qemu-backdoor.h"
+
+ #include "cpu.h"
+
+ #include <stdio.h>
+
+
+ void qemu_backdoor_init(uint64_t data_size)
+ {
+ printf("+ %"PRId64"\n", data_size);
+ }
+
+ void qemu_backdoor(uint64_t cmd, void *data)
+ {
+ /* Perform any endianess-wise loads to interpret the data */
+ uint64_t c = tswap64(cmd);
+ uint64_t d = tswap64(*(uint64_t*)data);
+ printf("-> %"PRIx64" :: %"PRIx64"\n", c, d);
+ }
+ EOF
+
+3. Build QEMU with the backdoor feature:
+
+ /path/to/qemu/configure --with-backdoor=/tmp/my-backdoor-qemu
+
+
+== Guest-side code ==
+
+1. Compile the corresponding guest-side interface library:
+
+ make -C /path/to/qemu-build/x86_64-linux-user/backdoor/guest
+
+2. Create a guest application to interact with the backdoor channel through the
+ interface declared in "backdoor/guest/qemu-backdoor.h":
+
+ cat > /tmp/my-backdoor-guest.c <<EOF
+ #include <stdio.h>
+ #include <errno.h>
+ #include <stdlib.h>
+ #include <string.h>
+ #include <qemu-backdoor.h>
+
+
+ int main()
+ {
+ /* This base path is only applicable to 'user' mode */
+ if (qemu_backdoor_init("/tmp/backdoor") != 0) {
+ fprintf(stderr, "error: qemu_backdoor_init: %s\n",
strerror(errno));
+ abort();
+ }
+
+ uint64_t vcmd = 0xcafe;
+ uint64_t vdata = 0xbabe;
+
+ printf("size: %"PRId64"\n", qemu_backdoor_data_size());
+ printf("sending cmd: 0x%"PRIx64" data: 0x%"PRIx64"\n", vcmd, vdata);
+
+ /* Get a pointer to the beginning of the data channel */
+ uint64_t * data = qemu_backdoor_data();
+
+ /* Write anything into the channel */
+ *data = vdata;
+ /* Invoke the channel */
+ qemu_backdoor(vcmd);
+ }
+ EOF
+
+3. Link the guest application against "libqemu-backdoor-guest.a":
+
+ gcc -o /tmp/my-backdoor-guest /tmp/my-backdoor-guest.c
/path/to/qemu-build/x86_64-linux-user/backdoor/guest/libqemu-backdoor-guest.a
-I/path/to/qemu/backdoor/guest
+
+
+== Running QEMU ==
+
+If you want to use QEMU's 'softmmu' mode:
+
+ /path/to/qemu-build/x86_64-softmmu/qemu-system-x86_64 -device backdoor
+ sudo /tmp/my-backdoor-guest # inside the VM
+
+If you want to use QEMU's 'user' mode:
+
+ /path/to/qemu-build/x86_64-linux-user/qemu-x86_64 -backdoor /tmp/backdoor
/tmp/my-backdoor-guest
+
+
+== Implementation details ==
+
+The backdoor channel is composed of two channels that are handled as
+memory-mapped files. The data channel is used to contain arbitrary data to
+communicate back and forth between the guest and QEMU. The control channel is
+used by the guest to read the size of the data channel and to write into it to
+signal that the data channel is ready to be used.
+
+When using the 'softmmu' mode, the backdoor communication channels are provided
+as a virtual device used through MMIO. The data channel acts as regular memory
+and the control channel intercepts all accesses to it to proxy them to the
+user-provided backdoor library.
+
+When using the 'user' mode, the backdoor communication channels are provided as
+regular files in the host system that the guest must 'mmap' into its address
+space. The data channel acts as regular memory and the 'mmap' of the control
+channel is intercepted in QEMU to establish if it's an 'mmap' for the control
+channel file. If that's the case, the memory that QEMU allocates for the guest
+is 'mprotect'ed to intercept all accesses to it performed by the guest and
proxy
+them to the user-provided backdoor library.
+
+Note that guest accesses to the device are automatically serialized by QEMU
into
+a single thread that handles the host-side backdoor callbacks. This means that
+its operation is thread-safe (as long as the command is written with a single
+guest instruction), and the 64 bits used to signal a backdoor command can be
used
+to index into different portions of the data channel so that multiple guest
+processes/threads can use the backdoor channel concurrently.
