Re: PowerBook 5xx instability fix.

[Nat Sloss <nathanialsloss%yahoo.com.au@localhost>]

On Sun, 27 Oct 2024 18:55:21 Martin Husemann wrote:
> On Sun, Oct 27, 2024 at 08:12:26AM +1100, Nathanial Sloss wrote:
> > Hi,
> > 
> > I've found that the PowerBook 5xx would crash randomly in pmap functions.
> >  It seems as though PTEs were referenced beyond the end of ram.
> > 
> > Unless there is a better way to handle this I'd like to commit the
> > attached patch.
> > 
> > I found the fix while limiting the available ram in the Booter.  Making
> > the ram smaller in the Booter the PowerBook never crashed.  But to lose
> > 1MB ram is wasteful so with the patch at least I only lose 48k.
> 
> What mmu_type does that machine have?
> 
> Can you add debug code to dump the generated PTEs during pmap_bootstrap
> (or can ddb show them? If not we probably should add a mach command to
> do that)
> 
> Could this be some bug in the msgbuf setup (machdep.c lines 294ff) ?
> 
> Martin

I've done some more investigating.  It seems it is only unstable when using an 
audio kernel.  I believe there is a possibility some of the pages at the top 
on memory are being used by the ASC/EASC.

The crashes are easily reproducible as simple as listing a directory while 
triggering a console bell.

I've only noticed this on the PowerBook520/520 with an EASC but I'm unaware of 
other machines which may be affected.

The attached patch fixes it and I'd like to commit it if there are no 
objections.

Best regards,

Nat

diff -r d60e61cd41cf sys/arch/mac68k/mac68k/pmap_bootstrap.c
--- a/sys/arch/mac68k/mac68k/pmap_bootstrap.c	Sat Mar 29 01:43:38 2025 +0000
+++ b/sys/arch/mac68k/mac68k/pmap_bootstrap.c	Sun Mar 30 11:22:33 2025 +1100
@@ -503,7 +503,7 @@ pmap_bootstrap(paddr_t nextpa, paddr_t f
 	physmem = m68k_btop(avail_remaining + nextpa - firstpa);
 
 	maxaddr = high[numranges - 1] - m68k_ptob(1);
-	high[numranges - 1] -= (m68k_round_page(MSGBUFSIZE) + m68k_ptob(1));
+	high[numranges - 1] -= (m68k_round_page(MSGBUFSIZE) + m68k_ptob(8));
 	avail_end = high[numranges - 1];
 	mem_size = m68k_ptob(physmem);
 	virtual_end = VM_MAX_KERNEL_ADDRESS;