Re: [Qemu-devel] Get only TCG code without execution

[Jamie Lokier]

Peter Maydell wrote:
> >  "guest binaries don't actually rely that much on the memory model."
> >
> > I think the reason is those guest binaries are single thread. Memory model 
> > is
> > important in multi-threaded case. BTW, our binary translator now can 
> > translate
> > x86 binary to ARM binary, and ARM has weaker memory model than x86.
> 
> Yes. At the moment this works for QEMU on ARM hosts because in
> system mode QEMU itself is single-threaded so the nastier interactions
> between multiple guest CPUs don't occur (just about every memory model
> defines that memory interactions within a single thread of execution
> behave in the obvious manner).

> I also had in mind that guest binaries
> tend to make fairly stereotypical use of things like LDREX/STREX
> rather than relying on obscure details like their interaction with
> plain load/stores.

As x86 doesn't use or need barrier instructions, when translating x86
to (say) run on ARM host, multi-threaded code that needs barriers
isn't easy to detect, so barriers may be required between every memory
access in the generated ARM code.

-- Jamie