Re: [Qemu-devel] Missing ARMv6 instructions?

[Jamie Lokier <jamie@shareable.org>]

Wolfgang Schildbach wrote:
> The way I understand this is that anyone who got ARM documentation with 
> the license that Paul mentioned, could not contribute patches that 
> implement v6 emulation. If, however, someone else (who has not signed such 
> a license) were to buy documentation about the ARMv6 architecture that 
> comes without a license (for example, the "ARM System Developer's Guide", 
> by Sloss, Symes, and Wright, Elsevier 2004), that someone should be able 
> to contribute (at least in those aspects of the ARM that are disclosed by 
> the book). In that case, ARM should not have any legal hooks except 
> copyright, and since at that point you are not using any of their code, 
> there is no case. However, I am not a lawyer, so don't take my word for 
> it.

I wonder about patents (and their validity).

A MIPS hardware implementation that I worked with had all the basic
MIPS integer instructions except one small group, on the ground that
those instructions were covered by a MIPS patent and so they dared not
include it.  It was the instruction used for unaligned memory word
access.  For that reason, we used a modified GCC specially for that
chip.

> I also find it hard to believe that building a product competitive
> to an ARM is possible using a description of the instruction set alone.

(I meant a description of the architectural behaviour... not just the
instruction set.  Compilers are moving beyond merely implementing
"user space" code, to optimising paths between security levels,
analysing memory orderings between threads, etc.  And for that matter,
so are decompilers.)

It's not possible.. yet.  Believe me, there are people working on it.

And it doesn't have to be competitive for ARM to have a reaction... as
we're seeing with software emulatation.

> That said, I agree with Paul that lobbying ARM to change their license is 
> probably a better route to go. I believe software emulators like qemu are 
> really in ARMs best interest since they support ARM development and 
> thereby increase the availability of software for ARM cores.

That makes sense to me.

-- Jamie