port-alpha/56200: TLB shootdown assertion in pmap_enter() L2-delref case

[thorpej%me.com@localhost]

>Number:         56200
>Category:       port-alpha
>Synopsis:       TLB shootdown assertion in pmap_enter() L2-delref case
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    port-alpha-maintainer
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sun May 23 15:30:00 +0000 2021
>Originator:     Jason Thorpe
>Release:        NetBSD 9.99.82
>Organization:
RISCy Business
>Environment:
NetBSD alpha-vm 9.99.82 NetBSD 9.99.82 (GENERIC-$Revision: 1.410 $) #1: Sat May 22 11:30:30 PDT 2021  thorpej@the-ripe-vessel:/space/src/sys/arch/alpha/compile/GENERIC.QEMU alpha

Configured for 32Mb of RAM.
>Description:
[ 2391.0239881] panic: kernel diagnostic assertion "tlbctx->t_pmap != NULL" failed: file "../../../../arch/alpha/alpha/pmap.c", line 934 
[ 2391.0239881] cpu0: Begin traceback...
[ 2391.0239881] alpha trace requires known PC =eject=
[ 2391.0239881] cpu0: End traceback...
Stopped in pid 8438.8438 (sh) at        netbsd:cpu_Debugger+0x4:        ret     
zero,(ra)
db{0}> trace
cpu_Debugger() at netbsd:cpu_Debugger+0x4
db_panic() at netbsd:db_panic+0xc8
vpanic() at netbsd:vpanic+0x108
kern_assert() at netbsd:kern_assert+0x70
pmap_tlb_shootnow.part.0() at netbsd:pmap_tlb_shootnow.part.0+0x3b0
pmap_enter_l2pt_delref() at netbsd:pmap_enter_l2pt_delref+0x7c
pmap_enter() at netbsd:pmap_enter+0x814
uvm_fault_internal() at netbsd:uvm_fault_internal+0x1884
trap() at netbsd:trap+0x51c
XentMM() at netbsd:XentMM+0x20
--- memory management fault (from ipl 0) ---
--- user mode ---
db{0}>
>How-To-Repeat:
I encountered this running ATF tests on 32Mb RAM configuration.
>Fix:
N/A