Re: issue with dom0_pvh on Xen 4.20

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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?

~Andrew