Re: [Qemu-devel] [PATCH] linux-user: Support explicit targets for PowerPC

[Aaron Sierra <asierra@xes-inc.com>]

----- Original Message -----
> From: "Laurent Vivier" <laurent@vivier.eu>
> Sent: Tuesday, November 7, 2017 3:24:52 AM

> Le 07/11/2017 à 03:50, Aaron Sierra a écrit :
>> Enable building PowerPC targets supporting a specific CPU, without
>> having to set QEMU_CPU via the environment. For example these build
>> targets (and many more) become available:
>> 
>> qemu-ppc.e500mc qemu-ppc.e500v2 qemu-ppc.e5500 qemu-ppc.e600
>> qemu-ppc.e6500
>> 
>> These (statically compiled) binaries have proven useful for
>> emulating PowerPC CPUs within Docker containers, where it's hard to
>> reliably define environment variables that are available for every
>> process.
>> 
> 
> An other idea would be to extract the default cpu from argv[0].
>
> We don't change the default CPU at compile time, but we check the binary
> name:
> - if it's qemu-ppc, let's use the default cpu for qemu-ppc
> - if it's not qemu-ppc, but something like qemu-ppc.XXX, let's set the
> CPU to XXX. For instance "qemu-ppc.e600" will be a shortcut for
> "qemu-ppc -cpu e600". [1]
> 
> I think it's easy to implement and don't change the default behavior of
> qemu. And you can use hardlink to define several binaries with different
> defaults (like busybox)

Laurent,

I had that thought, too, as being the cleanest and simplest to implement,
but it didn't work out as a practical solution. Having execution depend
on the name of the file would require a lot of tuning of magics, masks,
and interpreters in /proc/sys/fs/binfmt_misc to get the behavior that I
depend on. It would require one entry per binary.

With the implementation that I've proposed, I only need two fairly generic
entries (ppc and ppc64) stock from Ubuntu's qemu-user-static package.
With Docker it's trivial to bind <any_path>/qemu-ppc.e500mc to
/usr/bin/qemu-ppc-static within the container. Which gives me complete
control over how the container is emulated.

-Aaron S.

> 
> Thanks,
> Laurent
> 
> [1] something like:
> 
> -- a/linux-user/main.c
> +++ b/linux-user/main.c
> @@ -4165,6 +4165,7 @@ static void usage(int exitcode)
> static int parse_args(int argc, char **argv)
> {
>     const char *r;
> +    const char *cpuname;
>     int optind;
>     const struct qemu_argument *arginfo;
> 
> @@ -4179,6 +4180,11 @@ static int parse_args(int argc, char **argv)
>         }
>     }
> 
> +    cpuname = strrchr(argv[0], '.');
> +    if (cpuname && cpuname[1] != 0) {
> +        cpu_model = strdup(cpuname + 1);
> +    }
> +
>     optind = 1;
>     for (;;) {
>          if (optind >= argc) {