bug#58320: Hurd VM fails to boot on AMD EPYC (kvm-amd)

["Ludovic Courtès" <ludo@gnu.org>]

Hi!

As suggested by Samuel on IRC, I did that early on in kdb:

  debug traps /on

such that it would stop on each trap, hopefully allowing me to see why
exec is not starting.

--8<---------------cut here---------------start------------->8---
module 0: ext2fs --multiboot-command-line=${kernel-command-line} --host-priv-por                    
t=${host-port} --device-master-port=${device-port} --exec-server-task=${exec-tas                    
k} --store-type=typed --x-xattr-translator-records ${root} $(task-create) $(task                    
-resume)                                                                                            
module 1: exec /gnu/store/99sqiayswrxxb80331pl7jxin18wv28b-hurd-0.9-1.91a5167/hu                    
rd/exec $(exec-task=task-create)                                                                    
2 multiboot modules                                                                                 
task loaded: ext2fs --multiboot-command-line=root=device:hd0s1 root=3367134b-cfb                    
d-1e90-2f38-dfd13367134b gnu.system=/gnu/store/m66ccpdzdbcd3k2fdvyaj8cgmk23lybn-                    
system gnu.load=/gnu/store/m66ccpdzdbcd3k2fdvyaj8cgmk23lybn-system/boot --host-p                    
riv-port=1 --device-master-port=2 --exec-server-task=3 --store-type=typed --x-xa                    
ttr-translator-records device:hd0s1                                                                 
task loaded: exec /gnu/store/99sqiayswrxxb80331pl7jxin18wv28b-hurd-0.9-1.91a5167                    
/hurd/exec                                                                                          
                                                                                                    
start ext2fs: Hurd server bootstrap: ext2fs[device:hd0s1] execkernel: Page fault                    
 (14), code=6                                                                                       
Stopped at  0x1000:     pushl   0x4(%ebx)                                                           
>>>>> user space <<<<<                                                                              
0x1000(bfffff24,0,0,1160b,0)                                                                        
0x11627(bfffff9c,0,0,0,2)                                                                           
0x11bb()                                                                                            
db> show all threads                                                                                
    TASK        THREADS                                                                             
  0 gnumach (f5f7cf00): 7 threads:                                                                  
              0 (f5f7be18) .W..N. 0xc11dac04                                                        
              1 (f5f7bcd0) R..O..(idle_thread_continue)                                             
              2 (f5f7bb88) .W.ON.(reaper_thread_continue) 0xc12015d4                                
              3 (f5f7ba40) .W.ON.(swapin_thread_continue) 0xc11f8e2c                                
              4 (f5f7b8f8) .W.ON.(sched_thread_continue) 0                                          
              5 (f5f7b7b0) .W.ON.(io_done_thread_continue) 0xc1201f74                               
              6 (f5f7b668) .W.ON.(net_thread_continue) 0xc11db0a8                                   
  1 ext2fs (f5f7ce40): 6 threads:                                                                   
              0 (f5f7b520) .W.O.F(mach_msg_continue) 0                                              
              1 (f5f7b290) .W.O..(mach_msg_receive_continue) 0                                      
              2 (f5f7b148) .W.O..(mach_msg_receive_continue) 0                                      
              3 (f5f7b000) .W.O..(mach_msg_continue) 0                                              
              4 (f67d4e20) .W.O..(mach_msg_receive_continue) 0                                      
              5 (f67d4cd8) .W.O..(mach_msg_continue) 0                                              
  2 exec (f5f7cd80): (f5f7b3d8) R.....                                                              
--8<---------------cut here---------------end--------------->8---

Then lots of page faults with the same stack trace, seemingly endlessly:

--8<---------------cut here---------------start------------->8---
db> c                                                                                               
kernel: Page fault (14), code=6                                                                     
Stopped at  0x1000:     pushl   0x4(%ebx)                                                           
>>>>> user space <<<<<                                                                              
0x1000(bfffff24,0,0,1160b,0)                                                                        
0x11627(bfffff9c,0,0,0,2)                                                                           
0x11bb()                                                                                            
--8<---------------cut here---------------end--------------->8---

When I “debug traps /off” and continue, the startup process hangs as
normal, and at that point ‘show all threads’ no longer shows exec.

On a “working” VM, with traps enabled early on in the same way, I don’t
see any page fault until after exec, proc, auth, etc. have been started.

Thoughts?

Ludo’.