On Tue, Feb 03, 2004 at 04:11:43PM +0100, Maciej W. Rozycki wrote:
> I'm pretty sure the hazard is in both directions -- why? Because it's
> marked both in the "CP0 Data Used, Stage Used" and the "CP0 Data Written,
> Stage Available" column. I interpret that as a requirement for the CACHE
> instructions to "start using data" two instructions ahead and "finish
> writing data" two instructions after itself. If your assumption was true,
> I'd put the marking only in the former column. Of course that does not
> mean the table is correct, but I'd assume so, for safety, if not anything
I was just trying to explain why the PC variants used to work fine.
> > violated this hazard since almost beginning of the time, see
> > http://www.linux-mips.org/cvsweb/linux/arch/mips/mm/Attic/pg-r4k.S?rev=1.9
> > and I've not heared of any problems arising from this.
> Perhaps it wasn't really tested. Have you ever run the code on a PC
> variant? Has anyone else?
Yes, it has. Olivetti M700-10, around 2.2 or so. The code used at that
time in arch/mips/mm/r4xx0.c was not much different from what pg-r4k.c is
generating now that is it violates this hazard.
> > Any DECstations using the R400PC CPU variant?
> None. That would normally be a downgrade in memory throughput as the
> R2k/R3k DECstations used to have 64kB of I-cache + 64kB of D-cache,
> typically, and sometimes (with the 33MHz variant) even 128kB of D-cache.
> I suppose so -- without the "mips-pg-r4k-scache" patch my system is very
> unstable and the difference is essentially that in addition to
> Create_Dirty_Excl_SD there are additional Create_Dirty_Excl_D ones, that,
> apart from being a performance hit, shouldn't have any effect. I have to
> investigate it further yet.
Interesting, I was expecting somewhat better performance from the
combination of both. Anyway, what is now in CVS is tested on R4400SC V4.0.