mips64emul documentation: Technical details

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## Technical details

This page describes some of the internals of mips64emul.

- Overview
- Speed
- Networking
- Emulation of hardware devices
- Regression tests

### Overview

In simple terms, mips64emul is just a simple fetch-and-execute loop; an instruction is fetched from memory, and executed.

In reality, a lot of things need to be handled. Before each instruction is executed, the emulator checks to see if any interrupts are asserted which are not masked away. If so, then an INT exception is generated. Exceptions cause the program counter to be set to a specific value, and some of the system coprocessor's registers to be set to values signifying what kind of exception it was (an interrupt exception in this case).

Reading instructions from memory is done through a TLB, a translation lookaside buffer. The TLB on MIPS is software controlled, which means that the program running inside the emulator (for example an operating system kernel) has to take care of manually updating the TLB. Some memory addresses are translated into physical addresses directly, some are translated into valid physical addresses via the TLB, and some memory references are not valid. Invalid memory references cause exceptions.

After an instruction has been read from memory, the emulator checks which opcode it contains and executes the instruction. Executing an instruction usually involves reading some register and writing some register, or perhaps a load from memory (or a store to memory). The program counter is increased for every instruction.

Some memory references point to physical addresses which are not in the normal RAM address space. They may point to hardware devices. If that is the case, then loads and stores are converted into calls to a device access function. The device access function is then responsible for handling these reads and writes. For example, a graphical framebuffer device may put a pixel on the screen when a value is written to it, or a serial controller device may output a character to stdout when written to.

### Speed

There are two modes in which the emulator can run, a) a straight forward loop which fetches one instruction from emulated RAM and executes it (described in the previous section), and b) using dynamic binary translation.

Mode a is very slow. On a 2.8 GHz Intel Xeon host the resulting emulated machine is rougly equal to a 7 MHz R3000 (or a 3.5 MHz R4000). The actual performance varies a lot, maybe between 5 and 10 million instructions per second, depending on workload.

Mode b ("bintrans") is still to be considered experimental, but gives higher performance than mode a. It translates MIPS machine code into machine code that can be executed on the host machine on-the-fly. The translation itself obviously takes some time, but this is usually made up for by the fact that the translated code chunks are executed multiple times. To run the emulator with binary translation enabled, just add -b to the command line.

Only small pieces of MIPS machine code are translated, usually the size of a function, or less. There is no "intermediate representation" code, so all translations are done directly from MIPS to host machine code.

The default bintrans cache size is 16 MB, but you can change this by adding -DDEFAULT_BINTRANS_SIZE_IN_MB=xx to your CFLAGS environment variable before running the configure script, or by using the bintrans_size() configuration file option when running the emulator.

By default, an emulated OS running under DECstation emulation which listens to interrupts from the mc146818 clock will get interrupts that are close to the host's clock. That is, if the emulated OS says it wants 100 interrupts per second, it will get approximately 100 interrupts per real second.

There is however a -I option, which sets the number of emulated cycles per seconds to a fixed value. Let's say you wish to make the emulated OS think it is running on a 40 MHz DECstation, and not a 7 MHz one, then you can add -I 40000000 to the command line. This will not make the emulation faster, of course. It might even make it seem slower; for example, if NetBSD/pmax waits 2 seconds for SCSI devices to settle during bootup, those 2 seconds will take 2*40000000 cycles (which will take more time than 2*7000000).

The -I option is also necessary if you want to run deterministic experiments, if a mc146818 device is present.

Some emulators make claims such as "x times slowdown," but in the case of mips64emul, the host is often not a MIPS-based machine, and hence comparing one MIPS instruction to a host instruction doesn't work. Performance depends on a lot of factors, including (but not limited to) host architecture, host speed, which compiler and compiler flags were used to build mips64emul, what the workload is, and so on. For example, if an emulated operating system tries to read a block from disk, from its point of view the read was instantaneous (no waiting). So 1 MIPS in an emulated OS might have taken more than one million instructions on a real machine. Because of this, imho it is best to measure performance as the actual (real-world) time it takes to perform a task with the emulator.

### Networking

Running an entire operating system under emulation is very interesting in itself, but for several reasons, running a modern OS without access to TCP/IP networking is a bit akward. Hence, I feel the need to implement TCP/IP (networking) support in the emulator.

As far as I have understood it, there seems to be two different ways to go:

1. Forward ethernet packets from the emulated ethernet controller to the host machine's ethernet controller, and capture incoming packets on the host's controller, giving them back to the emulated OS. Characteristics are:
   - Requires direct access to the host's NIC, which means on most platforms that the emulator cannot be run as a normal user!
   - Reduced portability, as not every host operating system uses the same programming interface for dealing with hardware ethernet controllers directly.
   - When run on a switched network, it might be problematic to connect from the emulated OS to the OS running on the host, as packets sent out on the host's NIC are not received by itself. (?)

   or

2. Whenever the emulated ethernet controller wishes to send a packet, the emulator looks at the packet and creates a response. Packets that can have an immediate response never go outside the emulator, other packet types have to be converted into suitable other connection types (UDP, TCP, etc). Characteristics:
   - Each packet type sent out on the emulated NIC must be handled. This means that I have to do a lot of coding. (I like this, because it gives me an opportunity to learn about networking protocols.)
   - By not relying on access to the host's NIC directly, portability is maintained. (It would be sad if the networking portion of a portable emulator isn't as portable as the rest of the emulator.)
   - The emulator can be run as a normal user process, does not require root privilegies.
   - Connecting from the emulated OS to the host's OS should not be problematic.
   - The emulated OS will experience the network just as a single machine behind a NAT gateway/firewall would. The emulated OS is thus automatically protected from the outside world.

Other emulators that I have heard of seem to use the first one, if they support networking.

Since I have choosen the second kind of implementation, I have to write support explicitly for any kind of network protocol that should be supported. As of 2004-07-09, the following has been implemented and seems to work under at least NetBSD/pmax and OpenBSD/pmax under DECstation -D2 emulation:

- ARP requests sent out from the emulated NIC are interpreted, and converted to ARP responses. (This is used by the emulated OS to find out the MAC address of the gateway.)
- ICMP echo requests (that is the kind of packet produced by the ping program) are interpreted and converted to ICMP echo replies, regardless of the IP address. This means that running ping from within the emulated OS will always receive a response. The ping packets never leave the emulated environment.
- UDP packets are interpreted and passed along to the outside world. If the emulator receives an UDP packet from the outside world, it is converted into an UDP packet for the emulated OS. (This is not implemented very well yet, but seems to be enough for nameserver lookups, tftp file transfers, and NFS mounts using UDP.)
- TCP packets are interpreted one at a time, similar to how UDP packets are handled (but more state is kept for each connection). NOTE: Much of the TCP handling code is very ugly and hardcoded.
- RARP is not implemented yet. (I haven't needed it so far.)

The gateway machine, which is the only "other" machine that the emulated OS sees on its emulated network, works as a NAT-style firewall/gateway. It has a fixed IPv4 address of 10.0.0.254. An OS running in the emulator can thus have any 10.x.x.x address; a typical choice would be 10.0.0.1.

Inside emulated NetBSD or OpenBSD, running the following commands should configure the emulated NIC:

    	# ifconfig le0 10.0.0.1
    	# route add default 10.0.0.254
    	add net default: gateway 10.0.0.254

If you want nameserver lookups to work, you need a valid /etc/resolv.conf as well:

    	# echo nameserver 129.16.1.3 > /etc/resolv.conf

(But replace 129.16.1.3 with the actual real-world IP address of your nearest nameserver.)

Now, host lookups should work:

    	# host -a www.netbsd.org
    	Trying null domain
    	rcode = 0 (Success), ancount=2
    	The following answer is not authoritative:
    	The following answer is not verified as authentic by the server:
    	www.netbsd.org  86400 IN        AAAA    2001:4f8:4:7:290:27ff:feab:19a7
    	www.netbsd.org  86400 IN        A       204.152.184.116
    	For authoritative answers, see:
    	netbsd.org      83627 IN        NS      uucp-gw-2.pa.dec.com
    	netbsd.org      83627 IN        NS      ns.netbsd.org
    	netbsd.org      83627 IN        NS      adns1.berkeley.edu
    	netbsd.org      83627 IN        NS      adns2.berkeley.edu
    	netbsd.org      83627 IN        NS      uucp-gw-1.pa.dec.com
    	Additional information:
    	ns.netbsd.org   83627 IN        A       204.152.184.164
    	uucp-gw-1.pa.dec.com	172799 IN	A	204.123.2.18
    	uucp-gw-2.pa.dec.com	172799 IN	A	204.123.2.19

To transfer files via UDP, you can use the tftp program.

    	# tftp 12.34.56.78
    	tftp> get filename
    	Received XXXXXX bytes in X.X seconds
    	tftp> quit
    	#

or, to do it non-interactively (with ugly output):

    	# echo get filename | tftp 12.34.56.78
    	tftp> Received XXXXXX bytes in X.X seconds
    	tftp> #

This, of course, requires that you have put the file filename in the root directory of the tftp server (12.34.56.78).

It is also possible to run NFS via UDP. This is very useful if you want to share entire directory trees between the emulated environment and another machine. These instruction will work for FreeBSD, if you are running something else, use your imagination to modify them:

- On the server, add a line to your /etc/exports file, exporting the files you wish to use in the emulator:

      	/tftpboot -mapall=nobody -ro 123.11.22.33

  where 123.11.22.33 is the IP address of the machine running the emulator process, as seen from the outside world.

- Then start up the programs needed to serve NFS via UDP. Note the -n argument to mountd. This is needed to tell mountd to accept connections from unprivileged ports (because the emulator does not need to run as root).

      	# portmap
      	# nfsd -u       <--- u for UDP
      	# mountd -n

- In the guest OS in the emulator, once you have ethernet and IPv4 configured so that you can use UDP, mounting the filesystem should now be possible: (this example is for NetBSD/pmax or OpenBSD/pmax)

      	# mount -o ro,-r=1024,-w=1024,-U,-3 my.server.com:/tftpboot /mnt
          or
      	# mount my.server.com:/tftpboot /mnt

  If you don't supply the read and write sizes, there is a risk that the default values are too large. The emulator currently does not handle fragmentation/defragmentation of outgoing packets, so going above the ethernet frame size (1518) is a very bad idea. Incoming packets (reading from nfs) should work, though, for example during an NFS install.

The example above uses read-only mounts. That is enough for things like letting NetBSD/pmax or OpenBSD/pmax install via NFS, without the need for a CDROM ISO image. You can use a read-write mount if you wish to share files in both directions, but then you should be aware of the fragmentation issue mentioned above.

TCP is implemented to some extent, but should not be considered to be stable yet. It is enough to let NetBSD/pmax and OpenBSD/pmax install via ftp, though.

### Emulation of hardware devices

Each file in the device/ directory is responsible for one hardware device. These are used from src/machine.c, when initializing which hardware a particular machine model will be using. (I'll be using the name 'foo' as the name of the device in all these examples. This is pseudo code, it might need some modification to actually compile and run.)

Each device should have the following:

- An init function in dev_foo.c. It would typically look something like this:

      	void dev_foo_init(struct machine *machine, struct memory *mem,
      	    uint64_t baseaddr, int irq_nr)
      	{
      		struct foo_data *d;

      		d = malloc(sizeof(struct foo_data));
      		if (d == NULL) {
      			fprintf(stderr, "out of memory\n");
      			exit(1);
      		}
      		memset(d, 0, sizeof(struct foo_data));
      		d->irq_nr = irq_nr;

      		memory_device_register(mem, "foo", baseaddr,
      		    DEV_FOO_LENGTH, dev_foo_access, d);

      		/*  This should only be here if the device
      		    has a tick function:  */
      		machine_add_tickfunction(machine, dev_foo_tick, d,
      		    FOO_TICKSHIFT);
      	}

- The init function, the access function (described further down) and DEV_FOO_LENGTH should be defined in include/devices.h.

- At the top of dev_foo.c, the foo_data struct should be defined.

      	struct foo_data {
      		int	irq_nr;
      		/*  ...  */
      	}

- If foo has a tick function (that is, something that needs to be run at regular intervals) then FOO_TICKSHIFT and a tick function need to be defined as well:

      	#define FOO_TICKSHIFT		10

      	void dev_foo_tick(struct cpu *cpu, void *extra)
      	{
      		struct foo_data *d = (struct foo_data *) extra;

      		if (.....)
      			cpu_interrupt(cpu, d->irq_nr);
      		else
      			cpu_interrupt_ack(cpu, d->irq_nr);
      	}

- And last but not least, the device should have an access function. The access function is called whenever there is a load or store to an address which is in the device' memory mapped region.

      	int dev_foo_access(struct cpu *cpu, struct memory *mem,
      	    uint64_t relative_addr, unsigned char *data, size_t len,
      	    int writeflag, void *extra)
      	{
      		struct foo_data *d = extra;
      		uint64_t idata = 0, odata = 0;

      		idata = memory_readmax64(cpu, data, len);
      		switch (relative_addr) {
      		/* .... */
      		}

      		if (writeflag == MEM_READ)
      			memory_writemax64(cpu, data, len, odata);

      		/*  Perhaps interrupts need to be asserted or
      		    deasserted:  */
      		dev_foo_tick(cpu, extra);

      		/*  Return successfully.  */
      		return 1;
      	}

The return value of the access function has until 20040702 been a true/false value; 1 for success, or 0 for device access failure. A device access failure will be seen as a MIPS DBE exception from the CPU.

Right now I'm converting the devices to support arbitrary memory latency values. The return value is now the number of cycles that the read or write access took. A value of 1 means one cycle, a value of 10 means 10 cycles. Negative values are used for device access failures, and the absolute value of the value is then the number of cycles; a value of -5 means that the access failed, and took 5 cycles.

To be compatible with pre-20040702 devices, a return value of 0 is treated by the caller (in src/memory.c) as a value of -1.

### Regression tests

In order to make sure that the emulator actually works like it is supposed to, it must be tested. For this purpose, there is a simple regression testing framework in the tests/ directory.

NOTE: The regression testing framework is basically just a skeleton so far. Regression tests are very good to have. However, the fact that complete operating systems can run in the emulator indicate that the emulation is probably not too incorrect. This makes it less of a priority to write regression tests.

To run all the regression tests, type make regtest. Each assembly language file matching the pattern test_*.S will be compiled and linked into a 64-bit MIPS ELF (using a gcc cross compiler), and run in the emulator. If everything goes well, you should see something like this:

    	$ make regtest
    	cd tests; make run_tests; cd ..
    	gcc33 -Wall -fomit-frame-pointer -fmove-all-movables -fpeephole -O2
    		-mcpu=ev5 -I/usr/X11R6/include -lm -L/usr/X11R6/lib -lX11  do_tests.c
    		-o do_tests
    	do_tests.c: In function `main':
    	do_tests.c:173: warning: unused variable `s'
    	/var/tmp//ccFOupvD.o: In function `do_tests':
    	/var/tmp//ccFOupvD.o(.text+0x3a8): warning: tmpnam() possibly used
    		unsafely; consider using mkstemp()
    	mips64-unknown-elf-gcc -g -O3 -fno-builtin -fschedule-insns -mips64
    		-mabi=64 test_common.c -c -o test_common.o
    	./do_tests "mips64-unknown-elf-gcc -g -O3 -fno-builtin -fschedule-insns
    		-mips64 -mabi=64" "mips64-unknown-elf-as -mabi=64 -mips64"
    		"mips64-unknown-elf-ld -Ttext 0xa800000000030000 -e main
    		--oformat=elf64-bigmips" "../mips64emul"

    	Starting tests:
    	  test_addu.S (-a)
    	  test_addu.S (-a -b)
    	  test_clo_clz.S (-a)
    	  test_clo_clz.S (-a -b)
    	  ..
    	  test_unaligned.S (-a)
    	  test_unaligned.S (-a -b)

    	Done. (12 tests done)
    	    PASS:     12
    	    FAIL:      0

    	----------------

    	  All tests OK

    	----------------

Each test writes output to stdout, and there is a test_*.good for each .S file which contains the wanted output. If the actual output matches the .good file, then the test passes, otherwise it fails.

Read tests/README for more information.