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

[Juergen Gross <juergen.gross@xxxxxxxxxxxxxx>]

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
Is this the case when the guest itself uses single stepping? Initially the
debug trap shouldn't cause a VMEXIT, I think. And I'm not sure the hypervisor
will see a guest setting TF via an IRET.

I _have_ seen a debug trap in the guest after single stepping without db6
having the single step indicator set...

> - any DRn is being accessed
>
> So when your guest writes DR[0-3], debug registers should get
> restored (from their original zero values) and the guest would be
> permitted direct access to the hardware registers. Once context
> switched out, vmx_save_dr() ought to be saving the values
> (irrespective of DR7 contents, only depending upon
> v->arch.hvm_vcpu.flag_dr_dirty). During the next context
> switch in, they would get restored immediately if DR7 already has
> some breakpoint enabled, or again during first DR access if not.
Okay, I'll check that. A little test routine in my domU should be able to
verify that debug registers won't change under it's feet in case of no
activated events in db7.


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
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