Re: issue with dom0_pvh on Xen 4.20

[Manuel Bouyer <bouyer@xxxxxxxxxxxxxxx>]

On Tue, Sep 02, 2025 at 01:14:29PM +0100, Andrew Cooper wrote:
> On 02/09/2025 12:23 pm, Manuel Bouyer wrote:
> > On Tue, Sep 02, 2025 at 12:13:27PM +0100, Andrew Cooper wrote:
> >> On 02/09/2025 11:56 am, 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.
> >> Double and Triple faults occur when trying to deliver an exception
> >> generates an exception.  So while multiple faults are involved, only one
> >> instruction typically is.
> >>
> >> Is this an Intel or an AMD system?  One thing virt can do is break apart
> >> a triple fault, but the logic to do so is vendor specific.
> > it's an old intel system:
> > cpu0: "Intel(R) Xeon(R) CPU           X5650  @ 2.67GHz"
> > cpu0: Intel Xeon 36xx & 56xx, i7, i5 and i3 (686-class), 2667.30 MHz
> > cpu0: family 0x6 model 0x2c stepping 0x2 (id 0x206c2)
> >
> 
> Hmm.  Westmere EP.  Are you running with EPT active, or with Shadow Paging?

How do I know ?
Note that the same problem shows up on much newer systems: an i9, and a
Xeon W-2223. Both boots fine with the same NetBSD kernel and Xen 4.18 or 4.15.

I'm using this old Xeon for debug because this one has a serial console.

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