Re: [Qemu-devel] [4600] variable dynamic translation buffer size

[Fabrice Bellard]

Paul Brook wrote:
> On Wednesday 28 May 2008, Fabrice Bellard wrote:
> > +        code_gen_buffer = mmap(NULL, code_gen_buffer_size,
> > +                               PROT_WRITE | PROT_READ | PROT_EXEC,
> > +                               flags, -1, 0);
>
> I'd expect this to cause a significant performance regression on hosts that 
> have limited branch range (i.e. everything except x86). The mmap region is 
> almost never close to the application text segment, so all helper calls have 
> to be indirect.

I knew that when I did the patch, but having a dynamic size on x86 hosts 
prevails.

The other hosts can still implement hacks if they really need to, either 
by reserving a huge data in BSS as it was done before or by using the 
proc/x/maps entries to compute a possible mmap() address. IMHO I think 
this is unnecessary as helpers should be the exception.

Fabrice.