Re: [PATCH] MIPS: mm: Prevent a TLB shutdown on initial uniquification

[Maciej W. Rozycki]

On Tue, 11 Nov 2025, Thomas Bogendoerfer wrote:

> >  Can you please give it a try with your systems?
> 
> it's booting on my R4400 SGI Indy, but I see a lot of segmentation
> faults during system start. If I comment out r4k_tlb_uniquify() every-
> thing boots fine, which is kind of strange as there is a local_flush_tlb_all(),
> which should leave the TLB in the same stage.... No idea why, yet.

 Can you try the diagnostic patch below, which is what I used to verify 
this change, and report the entries produced?  Otherwise I wonder whether 
I haven't missed a barrier somewhere.

> > +
> > +	for (i = start, cnt = 0; i < tlbsize; i++, cnt++) {
> 
> shouldn't we read all TLB entries here ? 

 Strictly speaking it won't change anything as we call `write_c0_wired(0)' 
in `r4k_tlb_configure' before getting here and wired entries make no sense 
for KSEG0, so whatever would otherwise be there should not clash with our 
unique entries.  But I guess it won't hurt.  OTOH `ent' ought to be set up 
from `tlbsize' rather than `cnt' should the Wired register setting ever 
change.  I'll have a look, but I we need to know why the current version 
causes troubles with your system first.

 NB I wonder how this is supposed to work with mapped kernels, I thought 
the setup made with MAPPED_KERNEL_SETUP_TLB was meant to be permanent 
through the life of the system, hmm...

  Maciej
---
 arch/mips/mm/tlb-r4k.c |   10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

linux-mips-tlb-r4k-uniquify-fix-debug.diff
Index: linux-macro/arch/mips/mm/tlb-r4k.c
===================================================================
--- linux-macro.orig/arch/mips/mm/tlb-r4k.c
+++ linux-macro/arch/mips/mm/tlb-r4k.c
@@ -535,19 +535,23 @@ static void r4k_tlb_uniquify(void)
 	htw_stop();
 
 	for (i = start, cnt = 0; i < tlbsize; i++, cnt++) {
-		unsigned long vpn;
+		unsigned long entryhi, vpn;
 
 		write_c0_index(i);
 		mtc0_tlbr_hazard();
 		tlb_read();
 		tlb_read_hazard();
-		vpn = read_c0_entryhi();
+		entryhi = vpn = read_c0_entryhi();
 		vpn &= vpn_mask & PAGE_MASK;
 		tlb_vpns[cnt] = vpn;
+		printk("[%02x/%02x]: %08lx/%08lx\n", i, cnt, entryhi, vpn);
 	}
 
 	sort(tlb_vpns, cnt, sizeof(tlb_vpns[0]), r4k_vpn_cmp, NULL);
 
+	for (idx = 0; idx < cnt; idx++)
+		printk("[%02x]: %08lx\n", idx, tlb_vpns[idx]);
+
 	write_c0_entrylo0(0);
 	write_c0_entrylo1(0);
 
@@ -560,6 +564,8 @@ static void r4k_tlb_uniquify(void)
 			entryhi = UNIQUE_ENTRYHI(ent);
 			vpn = entryhi & vpn_mask & PAGE_MASK;
 
+			printk("[%02x/%02x/%02x]: %08lx/%08lx\n", i, idx, ent,
+			       vpn, tlb_vpns[idx]);
 			if (idx >= cnt || vpn < tlb_vpns[idx]) {
 				write_c0_entryhi(entryhi);
 				write_c0_index(i);