Re: issue with dom0_pvh on Xen 4.20

[Manuel Bouyer <bouyer@xxxxxxxxxxxxxxx>]

On Tue, Sep 02, 2025 at 02:22:29PM +0200, Juergen Gross wrote:
> On 02.09.25 12:56, Manuel Bouyer wrote:
> > On Tue, Sep 02, 2025 at 11:44:36AM +0100, Andrew Cooper wrote:
> > > On 02/09/2025 11:17 am, Manuel Bouyer wrote:
> > > > Hello,
> > > > I'm trying to boot a NetBSD PVH dom0 on Xen 4.20.
> > > > The same NetBSD kernel works fine with Xen 4.18
> > > > 
> > > > The boot options are:
> > > > menu=Boot netbsd-current PVH Xen420:dev hd0f:;load /netbsd-PVH 
> > > > console=com0 root=wd0f; multiboot /xen420-debug.gz dom0_mem=1024M 
> > > > console=com1 com1=38400,8n1 loglvl=all guest_loglvl=all 
> > > > gnttab_max_nr_frames=64 sync_console=1 dom0=pvh
> > > > 
> > > > and the full log from serial console is attached.
> > > > 
> > > > With 4.20 the boot fails with:
> > > > 
> > > > (XEN) *** Serial input to DOM0 (type 'CTRL-a' three times to switch 
> > > > input)
> > > > (XEN) Freed 664kB init memory
> > > > (XEN) d0v0 Triple fault - invoking HVM shutdown action 1
> > > > (XEN) *** Dumping Dom0 vcpu#0 state: ***
> > > > (XEN) ----[ Xen-4.20.2-pre_20250821nb0  x86_64  debug=y  Tainted:   C   
> > > >  ]----
> > > > (XEN) CPU:    7
> > > > (XEN) RIP:    0008:[<000000000020e268>]
> > > > (XEN) RFLAGS: 0000000000010006   CONTEXT: hvm guest (d0v0)
> > > > (XEN) rax: 000000002024c003   rbx: 000000000020e260   rcx: 
> > > > 00000000000dfeb7
> > > > (XEN) rdx: 0000000000100000   rsi: 0000000000103000   rdi: 
> > > > 000000000013e000
> > > > (XEN) rbp: 0000000080000000   rsp: 00000000014002e4   r8:  
> > > > 0000000000000000
> > > > (XEN) r9:  0000000000000000   r10: 0000000000000000   r11: 
> > > > 0000000000000000
> > > > (XEN) r12: 0000000000000000   r13: 0000000000000000   r14: 
> > > > 0000000000000000
> > > > (XEN) r15: 0000000000000000   cr0: 0000000000000011   cr4: 
> > > > 0000000000000000
> > > > (XEN) cr3: 0000000000000000   cr2: 0000000000000000
> > > > (XEN) fsb: 0000000000000000   gsb: 0000000000000000   gss: 
> > > > 0000000000000000
> > > > (XEN) ds: 0010   es: 0010   fs: 0000   gs: 0000   ss: 0010   cs: 0008
> > > > 
> > > > because of the triple fault the RIP above doens't point to the code.
> > > > 
> > > > I tracked it down to this code:
> > > >          cmpl    $0,%ecx                 ;       /* zero-sized? */      
> > > >  \
> > > >          je      2f                      ; \
> > > >          pushl   %ebp                    ; \
> > > >          movl    RELOC(nox_flag),%ebp    ; \
> > > > 1:      movl    %ebp,(PDE_SIZE-4)(%ebx) ;       /* upper 32 bits: NX */ 
> > > > \
> > > >          movl    %eax,(%ebx)             ;       /* store phys addr */  
> > > >  \
> > > >          addl    $PDE_SIZE,%ebx          ;       /* next PTE/PDE */     
> > > >  \
> > > >          addl    $PAGE_SIZE,%eax         ;       /* next phys page */   
> > > >  \
> > > >          loop    1b                      ; \
> > > >          popl    %ebp                    ; \
> > > > 2:                                      ;
> > > > 
> > > > there are others pushl/popl before so I don't think that's the problem
> > > > (in fact the exact same fragment is called just before with different
> > > > inputs and it doesn't fault). So the culprit it probably the write to 
> > > > (%ebx),
> > > > which would be 0x20e260
> > > > This is in the range:
> > > > (XEN)  [0000000000100000, 0000000040068e77] (usable)
> > > > so I can't see why this would be a problem.
> > > > 
> > > > Any idea, including how to debug this further, welcome
> > > 
> > > Even though triple fault's are aborts, they're generally accurate under
> > > virt, so 0x20e268 is most likely where things die.
> > 
> > but that's the RIP of the last fault, not the first one, right ?
> > 0x20e268 isn't in the text segment of the kernel, my guess is that the
> > first fault triggers an exception, but the exeption handler isn't set up yet
> > so we end up jumping to some random value.
> > 
> 
> What puzzles me is that:
> 
> - %cr2 is 0, so probably the first fault wasn't a page fault

AFAIK it can't be as we're still in real mode

> - RIP is %ebx + 8, so maybe the code was just clobbered by the loop?
> 
> Could it be the code has been moved to this location, or is about to
> be moved away afterwards?

No. RIP shouldn't end up there in any way. the assembly code is quite simple,
it's just a loop and I'm quite confident that we did enter the loop with
sane values

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