Re: [Qemu-devel] [PATCH] linux-user: Use the 5KEf processor for 64-bit emulation

[Peter Maydell]

On 20 November 2014 17:21, Maciej W. Rozycki <address@hidden> wrote:
>  It's not possible, the MIPS16 and microMIPS instruction sets are
> mutually exclusive as the same resource (the ISA bit or the LSB of the
> PC) is used to switch to either mode from the standard MIPS mode,
> depending on what a given processor implements.

Pity.

>  Besides it is worthwhile to have a processor that implements the
> microMIPS or the standard MIPS instruction set only, to trap unwanted
> mode switches in software that is supposed to use one of these
> instruction sets only (there is no such issue with the MIPS16
> instruction set as it it not fully self-contained and the presence of
> the standard MIPS instruction set is also required).

Indeed; for ARM "any" is an addition to the usual set of
CPU options, not a replacement for them.

>  The rest can probably be arranged although there are grey areas,
> specifically not all optional instruction sets have their opcodes
> defined in the microMIPS instruction set.  But for the user emulation
> mode that should probably be OK as long the artificial processor is not
> available for the system emulation mode.

Yes; for ARM we don't define the "any" CPU in system mode for
pretty much this reason.

>  Please note that as I suggested the right emulation could be chosen
> automagically based on ELF file header flags.  In fact I have a patch
> outstanding already down the queue that does some processor
> reconfiguration based on that, just like the Linux kernel would do.
> That approach could be further extended up to selecting among processors
> available, so e.g. a program that contains some microMIPS code would
> pick up a microMIPS-enabled processor and one including MIPS16 code
> would pick up one with the MIPS16 extension instead.

That would probably be a useful default. My views here are really
based on two things:
 * the binfmt-misc registration just starts QEMU with no -cpu options,
   so it's helpful if the default can run any binary that matches the
   ELF magic specification
 * the ifdef ladder in linux-user/main.c is kind of ugly :-)

thanks
-- PMM