Re: [Qemu-devel] tcg_register_jit_int() interface problem

[Peter Maydell]

On 16 July 2018 at 12:36, Aleksandar Markovic <address@hidden> wrote:
>> From: Laurent Vivier <address@hidden>
>> Le 16/07/2018 à 13:10, Aleksandar Markovic a écrit :
>> > For MIPS, the situation is different: Up until recently, MIPS had only
>> > one value for e_machine: that was EM_MIPS (used for both 32-bit and
>> > 64-bit MIPS systems). However, new nanoMIPS systems have a new value in
>> > that field: EM_NANOMIPS (that is to be applied for both 32-bit and
>> > 64-bit nanoMIPS systems).
>> >
>> >
>> > The situation described above represent a problem for us developing
>> > nanoMIPS support, forcing us to have different compilations (and
>> > executables) for MIPS and nanoMIPS.
>> >
>> >
>> > Is there a possibility to refactor tcg_register_jit_int() to receive
>> > "e_machine" value as an argument, instead of expecting preprocessor
>> > constant to be set?
>>
>> I don't know this par of code, but for PPC we have:
>>
>> #if TCG_TARGET_REG_BITS == 64
>> # define ELF_HOST_MACHINE EM_PPC64
>> #else
>> # define ELF_HOST_MACHINE EM_PPC
>> #endif
>>
>> So perhaps you could have something like:
>>
>> #if defined(HOST_NANOMIPS)
>> # define ELF_HOST_MACHINE EM_NANOMIPS
>> #else
>> # define ELF_HOST_MACHINE EM_MIPS
>> #endif
>
> The problem is, this means there will be separate executables for QEMU for
> MIPS and QEMU for nanoMIPS. Would that be acceptable from overall QEMU
> design point of view? Peter, Richard?

The assumption has previously been that there must be a value known
at compile time (because it would be the value of whatever is in the
ELF header for the QEMU executable itself, effectively). I'm not
sure why this doesn't work for MIPS still -- if you're building
on a host with the new nanoMIPS stuff and with that support enabled,
then report EM_NANOMIPS; if you're building a binary that works with
the older MIPS CPUs, report EM_MIPS ?

That said, if this doesn't work for MIPS, then it would be a trivial
refactor to allow the tcg per-target code to do
#define ELF_HOST_MACHINE mips_get_elf_host_machine()
so you could compute the value at runtime.

thanks
-- PMM