Re: [Qemu-devel] softmmu_header: ldb_kernel vs. ldsb_kernel

[Thiemo Seufer <ths@networkno.de>]

Clemens Kolbitsch wrote:
> hi everyone!
> i have a (maybe rather tricky) question:
> 
> why do you define 2 different inline load-functions in softmmu_header:
> 
> static inline int glue(glue(lds, SUFFIX), MEMSUFFIX)(target_ulong ptr)
> 
> vs.
> 
> static inline RES_TYPE glue(glue(ld, USUFFIX), MEMSUFFIX)(target_ulong ptr)
> 
> ??
> 
> Obviously this is for signed/unsigned access... but why do you need it? In 
> case there is a TLB miss, both functions call the same function (e.g. 
> __ldl_mmu) --> the return value cannot differ that much.
> 
> The only difference I see (that really matters) is how the bytes are copied to 
> the result-pointer (i.e. using movzbl vs. movsbl)... but that's it.

It is a cast. The generic C version for the other architectures makes
this more obvious.

> If there is some deeper reason behind that - could you please point that out 
> to me? And if there is such a thing, why is it not necessary for storing 
> (e.g. stb_kernel)??

A load (sign-)extends a value to register size, a store doesn't.

That's why you have e.g. in the MIPS instruction set LB, LBU and SB
but no SBU, it would do the same as SB.


Thiemo