Re: [Xen-devel] Debug-Registers in HVM domain destroyed

[Juergen Gross <juergen.gross@xxxxxxxxxxxxxx>]

On 14.02.2014 14:02, Jan Beulich wrote:
> > > > On 14.02.14 at 13:18, Juergen Gross <juergen.gross@xxxxxxxxxxxxxx> wrote:
> > On 14.02.2014 11:40, Jan Beulich wrote:
> > > > > > On 14.02.14 at 10:33, Juergen Gross <juergen.gross@xxxxxxxxxxxxxx> wrote:
> > > > Debug registers are restored on vcpu switch only if db7 has any debug events
> > > > activated. This leads to problems in the following cases:
> > > >
> > > > - db0-3 are changed by the guest before events are set "active" in db7. In
> > case
> > > >      of a vcpu switch between setting db0-3 and db7, db0-3 are lost. BTW:
> > setting
> > > >      db7 before db0-3 is no option, as this could trigger debug interrupts due
> > to
> > > >      stale db0-3 contents.
> > > >
> > > > - single stepping is used and vcpu switch occurs between the single step trap
> > > >      and reading of db6 in the guest. db6 contents (single step indicator)
> > are
> > > >      lost in this case.
> > > Not exactly, at least not looking at how things are supposed to work:
> > > __restore_debug_registers() gets called when
> > > - context switching in (vmx_restore_dr())
> > > - injecting TRAP_debug
Okay, db0-3 seem to be preserved. I did a test modifying the registers without
activating any debug traps. Even under heavy vcpu scheduling load everything
was fine.

> > Is this the case when the guest itself uses single stepping? Initially the
> > debug trap shouldn't cause a VMEXIT, I think.
> That looks like a bug, indeed - it's missing from the initially set
> exception_bitmap. Could you check whether adding this in
> construct_vmcs() addresses that part of the issue? (A proper fix
> would likely include further adjustments to the setting of this flag,
> e.g. clearing it alongside clearing the DR intercept.) But then
> again all of this already depends on cpu_has_monitor_trap_flag -
> if that's set on your system, maybe you could try suppressing its
> detection (by removing CPU_BASED_MONITOR_TRAP_FLAG from
> the optional feature set in vmx_init_vmcs_config())?
I've currently a test running with the attached patch (the bug was hit about
once every 3 hours, test is running now for about 4 hours without problem).
Test machine is running with Xen 4.2.3 hypervisor from SLES11 SP3.


Juergen

--
Juergen Gross                 Principal Developer Operating Systems
PBG PDG ES&S SWE OS6                   Telephone: +49 (0) 89 62060 2932
Fujitsu                                   e-mail: juergen.gross@xxxxxxxxxxxxxx
Mies-van-der-Rohe-Str. 8                Internet: ts.fujitsu.com
D-80807 Muenchen                 Company details: ts.fujitsu.com/imprint.html

Attachment: single-step.patch
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