Re: [PATCH v2 27/54] tcg/riscv: Require TCG_TARGET_REG_BITS == 64

[Richard Henderson <richard.henderson@linaro.org>]

On 4/12/23 22:18, Daniel Henrique Barboza wrote:
> 
> 
> On 4/10/23 22:04, Richard Henderson wrote:
>> The port currently does not support "oversize" guests, which
>> means riscv32 can only target 32-bit guests.  We will soon be
>> building TCG once for all guests.  This implies that we can
>> only support riscv64.
>>
>> Since all Linux distributions target riscv64 not riscv32,
>> this is not much of a restriction and simplifies the code.
> 
> Code looks good but I got confused about the riscv32 implications you cited.
> 
> Does this means that if someone happens to have a risc-v 32 bit host, with a
> special Linux sauce that runs on that 32 bit risc-v host, this person won't be
> able to build the riscv32 TCG target in that machine?

Correct.

At present, one is able to configure with such a host, and if one uses --target-list=x,y,z 
such that all of x, y or z are 32-bit guests the build should even succeed, and the result 
should probably work.

However, if one does not use --target-list in configure, the build will #error out here:

>> @@ -942,9 +913,6 @@ static void * const qemu_st_helpers[MO_SIZE + 1] = {
>>   #endif
>>   };
>> -/* We don't support oversize guests */
>> -QEMU_BUILD_BUG_ON(TCG_TARGET_REG_BITS < TARGET_LONG_BITS);
>> -

I am working on a patch set, not yet posted, which builds tcg/*.o twice, once for system 
mode and once for user-only.  At which point riscv32 cannot build at all.

I brought this patch forward from there in order to reduce churn.


r~