Re: [Xen-devel] [PATCH v2 09/15] xen/gntdev: use mmu_range_notifier_insert

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 11/1/19 1:48 PM, Jason Gunthorpe wrote:
> On Wed, Oct 30, 2019 at 12:55:37PM -0400, Boris Ostrovsky wrote:
>> On 10/28/19 4:10 PM, Jason Gunthorpe wrote:
>>> From: Jason Gunthorpe <jgg@xxxxxxxxxxxx>
>>>
>>> gntdev simply wants to monitor a specific VMA for any notifier events,
>>> this can be done straightforwardly using mmu_range_notifier_insert() over
>>> the VMA's VA range.
>>>
>>> The notifier should be attached until the original VMA is destroyed.
>>>
>>> It is unclear if any of this is even sane, but at least a lot of duplicate
>>> code is removed.
>> I didn't have a chance to look at the patch itself yet but as a heads-up
>> --- it crashes dom0.
> Thanks Boris. I spent a bit of time and got a VM running with a xen
> 4.9 hypervisor and a kernel with this patch series. It a ubuntu bionic
> VM with the distro's xen stuff.
>
> Can you give some guidance how you made it crash? 

It crashes trying to dereference mrn->ops->invalidate in
mn_itree_invalidate() when a guest exits.

I don't think you've initialized notifier ops. I don't see you using
gntdev_mmu_ops anywhere.

-boris


> I see the VM
> autoloaded gntdev:
>
> Module                  Size  Used by
> xen_gntdev             24576  2
> xen_evtchn             16384  1
> xenfs                  16384  1
> xen_privcmd            24576  16 xenfs
>
> And lsof says several xen processes have the chardev open:
>
> xenstored  819                 root   13u      CHR              10,53      
> 0t0      19595 /dev/xen/gntdev
> xenconsol  857                 root    8u      CHR              10,53      
> 0t0      19595 /dev/xen/gntdev
> xenconsol  857 860             root    8u      CHR              10,53      
> 0t0      19595 /dev/xen/gntdev
>
> But no crashing..
>
> However, I wasn't able to get my usual debug kernel .config to boot
> with the xen hypervisor, it crashes on early boot with:
>
> (XEN) Dom0 has maximum 8 VCPUs
> (XEN) Scrubbing Free RAM on 1 nodes using 8 CPUs
> (XEN) .done.
> (XEN) Initial low memory virq threshold set at 0x1000 pages.
> (XEN) Std. Loglevel: All
> (XEN) Guest Loglevel: All
> (XEN) *** Serial input -> DOM0 (type 'CTRL-a' three times to switch input to 
> Xen)
> (XEN) Freed 468kB init memory
> (XEN) d0v0 Unhandled page fault fault/trap [#14, ec=0002]
> (XEN) Pagetable walk from fffffbfff0480fbe:
> (XEN)  L4[0x1f7] = 0000000000000000 ffffffffffffffff
> (XEN) domain_crash_sync called from entry.S: fault at ffff82d080348a06 
> entry.o#create_bounce_frame+0x135/0x15f
> (XEN) Domain 0 (vcpu#0) crashed on cpu#0:
> (XEN) ----[ Xen-4.9.2  x86_64  debug=n   Not tainted ]----
> (XEN) CPU:    0
> (XEN) RIP:    e033:[<ffffffff82b9f731>]
> (XEN) RFLAGS: 0000000000000296   EM: 1   CONTEXT: pv guest (d0v0)
> (XEN) rax: fffffbfff0480fbe   rbx: 0000000000000000   rcx: 00000000c0000101
> (XEN) rdx: 00000000ffffffff   rsi: ffffffff84026000   rdi: ffffffff82cb4a20
> (XEN) rbp: ffffffff82407ff8   rsp: ffffffff82407da0   r8:  0000000000000000
> (XEN) r9:  0000000000000000   r10: 0000000000000000   r11: 0000000000000000
> (XEN) r12: 0000000000000000   r13: 1ffffffff0480fbe   r14: 0000000000000000
> (XEN) r15: 0000000000000000   cr0: 000000008005003b   cr4: 00000000003506e0
> (XEN) cr3: 0000000034027000   cr2: fffffbfff0480fbe
> (XEN) fsb: 0000000000000000   gsb: ffffffff82b61000   gss: 0000000000000000
> (XEN) ds: 0000   es: 0000   fs: 0000   gs: 0000   ss: e02b   cs: e033
>
> Which is surely some .config issue, but I didn't figure out what.
>
> Jason


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