Re: PPC440 tlbwe

[Nicholas Piggin]

On Mon Nov 13, 2023 at 6:38 AM AEST, BALATON Zoltan wrote:
> On Wed, 25 Oct 2023, Nicholas Piggin wrote:
> > On Wed Oct 25, 2023 at 2:01 AM AEST, BALATON Zoltan wrote:
> >> On Wed, 11 Oct 2023, Nicholas Piggin wrote:
> >>> On Tue Oct 10, 2023 at 10:41 PM AEST, BALATON Zoltan wrote:
> >>>> Hello,
> >>>>
> >>>> I have a question about tlbwe emulation in TCG for PPC440. This seems to
> >>>> be implemented in mmu_helper.c::helper_440_tlbwe which calls tlb_flush a
> >>>> lot flushing all TLB entries. This causes a big performance hit with
> >>>> sam460ex running AmigaOS which seems to do tlbwe on every task or context
> >>>> switch so it runs much slower than on other machines with a G4 CPU
> >>>> (pegasos2 and amigaone we also emulate) that use hardware TLB which does
> >>>> not have such problem in TCG. Even the embedded TLB emulation has 3
> >>>> versions for 40x, 440 and BookE 2.06 (I think that's e500) and at least
> >>>> the booke206 version uses flush_page instead of flushing everything. I
> >>>> don't know much about this so is there somebody with better understanding
> >>>> of this who can have a look at it and see if it can be optimised to
> >>>> resolve this problem or give some advice?
> >>>
> >>> It looks like 440 might be able to use the same page flushing as 4xx.
> >>>
> >>> This patch is only compile tested and I don't know the book E TLB stuff
> >>> too well but it may give you a starting point.
> >>
> >> A few more people gave this some testing and it seems to work and found no
> >> breakage so far. Do you plan to make it a porper patch and merge it
> >> sometimes?
> >
> > Performance is improved do you think? I can give it a try. I may try to
> > do the fix patches separately ahead of the performance patch. It would
> > be nice if those could be confirmed to fix a problem, but understand
> > it can be difficult to set up and hit TLB bugs.
>
> Nick,
>
> Are you still around?

Hey, a personal issue came up and I had to drop everything for a few
weeks (thanks to Daniel and others for stepping in to help). That's
over now, I'll try to start picking up the pieces again. Sorry if I
miss things...

> Too bad this patch hasn't been followed up on after 
> the reply showing performance increase with it but since it can also be 
> considered fixing actual bugs is there a chance for this to still be in 
> 8.2 release?

This may still be okay a a bug-fix. Separating the performance
improvement from the bug fix might actually not make it much simpler
but I'll see what it looks like.

Thanks,
Nick

>
> (Here's a link to the original trhread for reference:
> https://lists.nongnu.org/archive/html/qemu-ppc/2023-10/msg00334.html )
>
> Regards,
> BALATON Zoltan