Frequently Asked Questions

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Revision as of 20:32, 27 May 2007 by Stever (talk | contribs) (Running Related)
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Device Related

How can I see the packets on the ethernet link?
  • By creating a Etherdump object, setting it's file parameter, and setting the dump parameter on the EtherLink to it. This is easily accomplished with our example configuration by adding the command line option --etherdump=<filename>. The resulting file will be named <file> and be in a standard pcap format.

OS/Linux Related

What is a disk image?
  • A disk image is just a raw data file...we use disk images to fake a real hard disk. You can use the script that is provided in our distribution of linux-dist to create a blank image of arbitrary size.
How do I add files to a disk image?
  • Using either sudo or the root account run /bin/mount -o loop,offset=32256 /z/foo.img /mount/point. You can then copy the desired files to the image. Remember to unmount it before running the simulator with /bin/umount /mount/point or you may get unexpected results. This is a hack-y method, what you should do to add new binaries to M5 is modify linux-dist and place it in that build structure.
Can I use MIPS SDE Lite to cross-compile for M5?
  • The MIPS SDE Lite package is a very good tool but unfortunately it will not work if you would like to cross-compile a MIPS/Linux binary and run it on M5. The MIPS SDE package contains some SDE-specific startup routines and glibc calls that M5 will not be able to support. The cross-compiler solution we found to work is crosstool. For MIPS, gcc 3.4.4 and glibc 2.3.5 seems to work.

Running Related

I compiled an executable to run in syscall emulation mode but it doesn't work.
  • If M5 seems to initialize OK, but the CPU never fetches any instructions, it may be because your executable is dynamically linked. M5 does not support dynamic linking (shared libraries) in syscall emulation mode. You must recompile the executable and have it statically linked. With gcc, just add the "-static" flag to the command line.
How many CPUs can M5 run?
  • There is no inherent limit in M5 (other than simulation speed). In SE mode there are no obstacles to simulating as many CPUs as you want. However, in FS mode, the real-world Alpha platform we model (Tsunami) only has support for 4 processors. We made some additions to the platform both in the simulator and in Linux to support a variant of Tsunami (not corresponding to any real system) that can take up to 64 processors. Simulating 64 processors will be quite slow, and the Linux scheduler doesn't seem particularly good at scheduling a large number of processors. Take a look at the Download page for our patches.
How do I see what options I can set?
  • Using the '-h' flag will show what options M5 can take in general. Running m5.debug -h (or any m5 binary variant) will list all options that are available based on M5's internal options and the options defined in the .py file.
How do I run SPLASH benchmarks on M5?
How do I run multiprogram workloads on M5?
How do I use the sampler?

The sampler from the previous version of M5 has been replaced with functionality via Python. See the Sampling documentation for details.

Debugging and Error Related

When running my program I get an error "Error: syscall <syscall name here> unimplemented."
  • That means that one of the libraries that you are linking with uses a syscall that is not emulated. You can do a man on the syscall name to see what the syscall is supposed to do and then try to implement at least whatever functionality your application needs. Or you can try the quick & dirty approach, which is to change the function pointer for syscall in arch/<arch>/<os>/<os>.cc from unimplementedFunc to ignoreFunc, which will make it print a warning and keep going rather than terminate with an error. No guarantees that your program won't crash because of this though.
How do I access reference counted pointers in GDB?
  • Objects such as DynInsts are reference counted, making it slightly harder to obtain the data inside. In gdb you must access them through the pointer that is stored in the ref counted pointer, which is called data. Thus given a ref counted pointer ptr, in gdb you would say (gdb) ptr->data to get the pointer to the actual object.
I get an error "undefined reference to `void foo()'" when the compiler is doing the final linking.
  • This is due to having a function that is declared but never defined. Either you forgot to define it, or are not compiling in the file that defines it. In the case of templated code, you may be including the wrong file or you may not have manually instantiated a templated class that needs to be manually instantiated.
When I'm running in SE mode I get warnings about unimplemented or ignored system calls or trapping modes.
  • M5 does not support IEEE FP floating point traps (underflow, overflow, etc.) and as a result doesn't bother supporting the system calls that enable/disable these traps or set the corresponding trap handlers. It's pretty unlikely your application relies on them (we haven't seen one yet that does). As long as everything else seems to work you can ignore the warning.


Where are the classes for each ISA instruction? Where are the execute() methods for the StaticInst class?
  • Both the classes and the execute() methods are generated through Python upon building any version of M5. For example, After building ALPHA_SE, they will be located in the build/ALPHA_SE/arch/alpha/ folder. The key files are decoder.hh, (which describe the ISA instructions), and * (which describe the execute() methods). The definitions for both exist in the .isa files found in src/arch/*/isa/, which are processed by src/arch/ to generate the previously mentioned .hh/.cc files.
Is fast-forwarding supported in SMT mode?
  • It is not currently supported. The SimpleCPU doesn't support SMT, so it doesn't support fast-forwarding in SMT mode. However it should be feasible given some hacking on the SimpleCPU or the Sampler.
I've created a new file, how do I get SCons to compile it.
  • Add the file to the source list in the SConscript in the current directory or (if there is none) the closest parent directory.
I've got a new SimObject compiled but I can't use it.
  • You need to both have the building macros in your C++ file (take a look at any of the other objects for examples) and a corresponding Python file (take a look at src/python/m5/objects/* for examples). (In some future version we will get rid of the C++ macros and autogenerate the needed C++ code from the Python description... we're not there yet though.) You also need to edit src/python/m5/objects/ to include your Python file.
How do I use a normal variable in a statistic formula?
  • This is not supported. Just create a Scalar<> statistic that does the same thing as your normal variable and use that in the formula instead.
What are all these *_impl.hh files?
  • There is a lot of templated code used within M5, and these *_impl.hh are used to make it a little easier to organize things. Normally template functions must be entirely included in the header file in order to not require the programmer to manually instantiate the copies of the template functions. However, when you have an entire class that's templated, the header files quickly become bloated and too big, except for small helper classes. Thus we put the declarations in the header file as normal, the definitions in the *_impl.hh file, and the manual instantiations in the *.cc file. This makes it easier to sort out the instantiations from the definitions. Also if there are only a few templated functions inside a non-templated class, it may be possible to include the functions in the *_impl.hh file and not have to manually instantiate the functions. You just need to include the *_impl.hh file in any .cc files that use the templated function. See mem/packet{.hh|_impl.hh|.cc} for an example.
What if SCons complains that it can't find a file I just deleted?
  • Delete the scons.dblite file in the m5 directory.
Which config files are used for syscall emulation? Full system mode?

configs/example/ is a sample configuration file for syscall emulation. Similarly, configs/example/ is a sample configuration for full system simulations. Both these files include files in the configs/common/ directory.

Where does the stack and program arguments get setup for a process in Syscall Emulation mode?

A good point of reference for this is the "argsInit" function in the src/sim/ file. For Syscall Emulation, each process is given a "LiveProcess" object and that function initializes the arguments to that process and also sets up the initial stack for that process. You'll also notice that the architecture specific Process objects (e.g. AlphaLinuxLiveProcess found in arch/alpha/linux/process.hh) derive from the LiveProcess object too.