Re: [Qemu-devel] Adding SMP support for Sparc Target

[Damien Hilloulin]

Le 06/11/2014 16:27, Artyom Tarasenko a écrit :
> Hello Damien,
>
> On Thu, Nov 6, 2014 at 8:38 AM, Damien Hilloulin
> <address@hidden> wrote:
> > Hello everyone,
> >
> > I'm a newcomer in QEMU and my goal would be to port an existing system
> > simulator using another emulator to QEMU.
> > Some work has already been done, and Sparc has been the main target so far
> > because of its simplicity (and because we have a very good support for Sparc
> > with the other emulator).
> > QEMU is great, open-source (contrary to the other emulator we have been
> > using in the past), and that's why we are aiming at using it.
> >
> > However, it seems that the Sparc targets doesn't really support SMP/CMT as
> > of now. So I am considering two possibilities:
> > - adding SMP support in QEMU for the Sparc targets (and contribute it to
> > QEMU :) )
> Do you mean a) emulating multiple guest cores on in a single host
> thread, or b) emulating multiple guest cores in multiple host threads?
a) Would be enough for us (but b) would be amazing) !
> The former (a) should be relative easy for a sun4m platform: just have
> to put the CPUs at the proper place in the system bus and fill the CPU
> Module Ids (MIDs) with the proper data.
Could you please explain it with some more details? I don't really know 
how to do that (yet)...
We are really interested in such a support for Sparc64, so I think that 
it would be for sun4u machines only.
Would the changes would be the same or would there be more work?

> It would bring no performance increase though. In fact the guest OS
> would likely run slower because the speed of an emulated CPU would
> decrease like 1/N, and utilization of multiple CPUs by a guest OS is
> probably scale like ~ log N, where N is the number of CPUs emulated.
>
> If you mean b), things get more complicated because TCG can currently
> utilize just one host thread. There was an attempt to do utilize
> multiple threads for an ARM target:
> http://sourceforge.net/p/coremu/home/Home
>
> It would be interesting to hear what the TCG experts would say. Adding
> Richard to CC.
>
> Artyom
Speeding up the simulation would be really interesting for us, but would 
be in the next iteration of changes. Would be great if we had both the 
features and speed :)

Thanks for your help, it seems really promising to me!

Damien.