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 <bou...@antioche.eu.org>
NetBSD: 26 ans d'experience feront toujours la difference
--
