Re: dom0 PV looping on search_pre_exception_table()

[Manuel Bouyer <bouyer@xxxxxxxxxxxxxxx>]

On Wed, Dec 09, 2020 at 04:00:02PM +0000, Andrew Cooper wrote:
> [...]
> >> I wonder if the LDT is set up correctly.
> > I guess it is, otherwise it wouldn't boot with a Xen 4.13 kernel, isn't it ?
> 
> Well - you said you always saw it once on 4.13, which clearly shows that
> something was wonky, but it managed to unblock itself.
> 
> >> How about this incremental delta?
> > Here's the output
> > (XEN) IRET fault: #PF[0000]                                                 
> >    
> > (XEN) %cr2 ffff820000010040, LDT base ffffc4800000a000, limit 0057          
> >    
> > (XEN) *** pv_map_ldt_shadow_page(0x40) failed                               
> >    
> > (XEN) IRET fault: #PF[0000]                                                 
> >    
> > (XEN) %cr2 ffff820000010040, LDT base ffffc4800000a000, limit 0057          
> >    
> > (XEN) *** pv_map_ldt_shadow_page(0x40) failed                               
> >    
> > (XEN) IRET fault: #PF[0000]                                                 
> 
> Ok, so the promotion definitely fails, but we don't get as far as
> inspecting the content of the LDT frame.  This probably means it failed
> to change the page type, which probably means there are still
> outstanding writeable references.
> 
> I'm expecting the final printk to be the one which triggers.

It's not. 
Here's the output:
(XEN) IRET fault: #PF[0000]                                                    
(XEN) %cr2 ffff820000010040, LDT base ffffbd000000a000, limit 0057             
(XEN) *** LDT: gl1e 0000000000000000 not present                               
(XEN) *** pv_map_ldt_shadow_page(0x40) failed                                  
(XEN) IRET fault: #PF[0000]                                                    
(XEN) %cr2 ffff820000010040, LDT base ffffbd000000a000, limit 0057             
(XEN) *** LDT: gl1e 0000000000000000 not present                               
(XEN) *** pv_map_ldt_shadow_page(0x40) failed                                  
(XEN) IRET fault: #PF[0000]                                                    
(XEN) %cr2 ffff820000010040, LDT base ffffbd000000a000, limit 0057          
(XEN) *** LDT: gl1e 0000000000000000 not present
(XEN) *** pv_map_ldt_shadow_page(0x40) failed
(XEN) IRET fault: #PF[0000]
(XEN) %cr2 ffff820000010040, LDT base ffffbd000000a000, limit 0057
(XEN) domain_crash called from extable.c:219
(XEN) Domain 0 (vcpu#0) crashed on cpu#0:
(XEN) ----[ Xen-4.15-unstable  x86_64  debug=y   Tainted:   C   ]----
(XEN) CPU:    0
(XEN) RIP:    0047:[<00007f7f5dc007d0>]
(XEN) RFLAGS: 0000000000000202   EM: 0   CONTEXT: pv guest (d0v0)
(XEN) rax: ffff82d04038c309   rbx: 0000000000000000   rcx: 000000000000e008
(XEN) rdx: 0000000000010086   rsi: ffff83007fcb7f78   rdi: 000000000000e010
(XEN) rbp: 0000000000000000   rsp: 00007f7fffcfc8d0   r8:  0000000e00000000
(XEN) r9:  0000000000000000   r10: 0000000000000000   r11: 0000000000000000
(XEN) r12: 0000000000000000   r13: 0000000000000000   r14: 0000000000000000
(XEN) r15: 0000000000000000   cr0: 0000000080050033   cr4: 0000000000002660
(XEN) cr3: 0000000079cdb000   cr2: ffffbd000000a040
(XEN) fsb: 0000000000000000   gsb: 0000000000000000   gss: ffffffff80cf2dc0
(XEN) ds: 0023   es: 0023   fs: 0000   gs: 0000   ss: 003f   cs: 0047
(XEN) Guest stack trace from rsp=00007f7fffcfc8d0:
(XEN)    0000000000000001 00007f7fffcfcde8 0000000000000000 0000000000000000
(XEN)    0000000000000003 000000000e200040 0000000000000004 0000000000000038
(XEN)    0000000000000005 0000000000000008 0000000000000006 0000000000001000
(XEN)    0000000000000007 00007f7f5dc00000 0000000000000008 0000000000000000
(XEN)    0000000000000009 000000000e201cd0 00000000000007d0 0000000000000000
(XEN)    00000000000007d1 0000000000000000 00000000000007d2 0000000000000000
(XEN)    00000000000007d3 0000000000000000 000000000000000d 00007f7fffcfd000
(XEN)    00000000000007de 00007f7fffcfc9d0 0000000000000000 0000000000000000
(XEN)    6e692f6e6962732f 0000000000007469 0000000000000000 0000000000000000
(XEN)    0000000000000000 0000000000000000 0000000000000000 0000000000000000
(XEN)    0000000000000000 0000000000000000 0000000000000000 0000000000000000
(XEN)    0000000000000000 0000000000000000 0000000000000000 0000000000000000
(XEN)    0000000000000000 0000000000000000 0000000000000000 0000000000000000
(XEN)    0000000000000000 0000000000000000 0000000000000000 0000000000000000
(XEN)    0000000000000000 0000000000000000 0000000000000000 0000000000000000
(XEN)    0000000000000000 0000000000000000 0000000000000000 0000000000000000
(XEN)    0000000000000000 0000000000000000 0000000000000000 0000000000000000
(XEN)    0000000000000000 0000000000000000 0000000000000000 0000000000000000
(XEN)    0000000000000000 0000000000000000 0000000000000000 0000000000000000
(XEN)    0000000000000000 0000000000000000 0000000000000000 0000000000000000
(XEN) Hardware Dom0 crashed: rebooting machine in 5 seconds.

-- 
Manuel Bouyer <bouyer@xxxxxxxxxxxxxxx>
     NetBSD: 26 ans d'experience feront toujours la difference
--