Re: [Xen-devel] [ARM] gvirt_to_maddr fails when DomU is created

[Volodymyr Babchuk <vlad.babchuk@xxxxxxxxx>]

Hi Julien,

On Tue, 27 Nov 2018 at 21:40, Julien Grall <julien.grall@xxxxxxx> wrote:
> > After creating domU, I'm seeing lots of this messages from hypervisor:
> >
> > (XEN) p2m.c:1442: d1v0: gvirt_to_maddr failed va=0xffff80000efc7f0f
> > flags=0x1 par=0x809
> > (XEN) p2m.c:1442: d1v0: gvirt_to_maddr failed va=0xffff80000efc7f00
> > flags=0x1 par=0x809
> > (XEN) p2m.c:1442: d1v0: gvirt_to_maddr failed va=0xffff80000efc7f0f
> > flags=0x1 par=0x809
> >
> > Interestingly, I'm getting them from both Dom0 and DomU:
> >
> > (XEN) p2m.c:1442: d0v0: gvirt_to_maddr failed va=0xffff80003efd7f0f
> > flags=0x1 par=0x809
> > (XEN) p2m.c:1442: d1v0: gvirt_to_maddr failed va=0xffff80000efc7f0f
> > flags=0x1 par=0x809
> >
> > But only after DomU is created.
> >
> > I attached GDB and found that this is caused by update_runstate_area:
> >
> > (gdb) bt
> > #0  get_page_from_gva (v=0x80005dbe2000, v@entry=0x22f2c8 <schedule+1236>,
> >      va=va@entry=18446603337277996815, flags=flags@entry=1) at p2m.c:1440
> > #1  0x000000000024e320 in translate_get_page (write=true, linear=true,
> > addr=18446603337277996815,
> >      info=...) at guestcopy.c:37
> > #2  copy_guest (buf=buf@entry=0x80005dbe20d7,
> > addr=addr@entry=18446603337277996815, len=len@entry=1,
> >      info=..., flags=flags@entry=6) at guestcopy.c:69
> > #3  0x000000000024e45c in raw_copy_to_guest (to=to@entry=0xffff80003efd7f0f,
> >      from=from@entry=0x80005dbe20d7, len=len@entry=1) at guestcopy.c:110
> > #4  0x00000000002497b4 in update_runstate_area
> > (v=v@entry=0x80005dbe2000) at domain.c:287
> > #5  0x0000000000249eb8 in context_switch (prev=prev@entry=0x80005dbe2000,
> >      next=next@entry=0x80005bf3c000) at domain.c:344
> > #6  0x000000000022f2c8 in schedule () at schedule.c:1583
> > #7  0x0000000000232c10 in __do_softirq
> > (ignore_mask=ignore_mask@entry=0) at softirq.c:50
> > #8  0x0000000000232ca4 in do_softirq () at softirq.c:64
> > #9  0x0000000000258254 in leave_hypervisor_tail () at traps.c:2302
> >
> > This issue is encountered on QEMU-ARMv8. Dom0 kernel is Linux 4.19.0
> > My XEN master is at d8ffac1f7 "xen/arm: gic: Remove duplicated comment
> > in do_sgi"
> >
> > The same setup worked perfectly with Xen 4.10.2
>
> The message is only printed in debug build. Do you have CONFIG_DEBUG
> enabled?
>
Yes, I do.

> update_runstate_area is using a guest virtual address to update the vCPU
> runstate. It blindly assumes the vCPU runstate will always be mapped in
> stage-1 page-tables. However, if KPTI (Kernel Page Table Isolation) is
> enabled the kernel address space (and therefore the vCPU runstate) will
> not be mapped when running at EL0.
I tried to disable KPTI for both Dom0 and DomU kernels (with nopti
option) and this didn't helped at all.
I can verify, that kernel does not print "CPU features: detected:
Kernel page table isolation (KPTI)", but that's all.

Strangely, I'm starting to see this messages only after I create DomU.
If this really would be triggered
by KPTI, then I should see those errors right from the boot, right?

--
WBR Volodymyr Babchuk aka lorc [+380976646013]
mailto: vlad.babchuk@xxxxxxxxx

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