[vbox-dev] Linux-4.12-rc1 and rc2 not working as vbox guest ?

[Frank Mehnert]

Hi Hans,

On Donnerstag, 1. Juni 2017 14:22:29 CEST Hans de Goede wrote:
> > Again, this is not a VirtualBox bug although this panic will probably not
> > happen on real hardware.
>
> [...]
> 
> No as for the disagree-ing with this not being a VirtualBox bug,
> even with 4.11 the oops is a problem and is IMHO a VirtualBox bug,

why do you think it's a VirtualBox bug?

> I was sorta surprised when you said earlier in this thread that this
> is normal and can be ignored. Oopses are never normal and should never
> be ignored. Fedora / RHEL users will get a notification that the kernel
> has hit a bug as soon as they login because of the oops, and oopses
> are being actively tracked for people who opt-in to reporting crashes
> to our backtrace server:
> 
> https://retrace.fedoraproject.org/faf/problems/
> 
> Now maybe this really is 2 kernel bugs, 1. The oops turning into a
> hang with 4.12 and 2. The oops happening at all, but an oops is never
> normal and really must be fixed. In case you believe that the oops
> happening at all is also a kernel bug please file a bug for that
> too and cross-reference the 2.

I think so. The Linux kernel oops is about a wrong expectation of the
kernel: VirtualBox exposes the size of the XSAVE area (same value as
on the host, 0x3c0 = 960 bytes). VBox does not expose bits 0...3 of
the eax register (XSAVEOPT, XSAVEC/XRSTOR, XGETBV, XSAVES/XRSTORS
all not available). In general it's all about do_extra_xstate_size_checks().
That function is a bit hard to follow without any additional debug
output. All I can see is that

  fpu_kernel_xstate_size = 0x440 while
  paranoid_xstate_size   = 0x240

and therefore the XSTATE_WARN_ON() statement triggers. As I said, the
kernel expects different values in the CPUID registers and warns about
mismatched expectations. I'm 99% sure that the kernel would run correctly
if this warning is just ignored. The warning is paranoia.

That XSAVE stuff is work in progress and future versions of VirtualBox
will change the exposed features.

I will create bug reports.

Kind regards,

Frank
-- 
Dr.-Ing. Frank Mehnert | Software Development Director, VirtualBox
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