Re: [Qemu-devel] Debian 7.8.0 SPARC64 on qemu - anything i can do to speedup the emulation?

[Richard Henderson <rth@twiddle.net>]

On 08/26/2015 09:17 AM, Artyom Tarasenko wrote:
> After some debugging I think it's caused by memory faults. On every
> MMU miss / access fault
> TB is re-translated multiple times till the faulting instruction is found.

That shouldn't happen.  Are you certain it's not multiple MMU misses/faults?

> AFAICT we produce data/access faults only on load/store instructions, i.e.
> if GET_FIELD(insn, 0, 1)  == 3. Can this knowledge be used to reduce
> the number of re-translations?

No.

 From the fault, we have a host address where the fault occured.  We then 
retranslate the TB looking for what guest address corresponds to the code 
generated at the host address.  This is a one-pass process, not the multiple 
passes you seem to be imagining.  It also means we can't skip non-memory insns 
during retranslation, as the host addresses would no longer line up.

That said, sun4u is a software managed tlb, which requires *lots* more extra 
faults than a hardware managed tlb.  In the later case, we can perform the page 
table lookup and then continue the memory instruction without faulting.

I think that implementing sun4v, with (most of) the hypervisor actually within 
qemu, is the only way to get good performance for Sparc.

Anyway, this sort of setup is exactly what I did for Alpha.  The PALcode 
(hypervisor-ish) layer used for qemu looks nothing like the PALcode layer used 
for real hardware.


r~