Re: [PATCH v4 63/75] x86/sev-es: Handle #DB Events

Peter Zijlstra
2020-07-15T09:51:54Z

On Wed, Jul 15, 2020 at 11:13:37AM +0200, Joerg Roedel wrote:
> On Wed, Jul 15, 2020 at 10:47:52AM +0200, Peter Zijlstra wrote:
> > On Tue, Jul 14, 2020 at 02:09:05PM +0200, Joerg Roedel wrote:
> >
> > > @@ -1028,6 +1036,16 @@ DEFINE_IDTENTRY_VC_SAFE_STACK(exc_vmm_communication)
> > > struct ghcb *ghcb;
> > >
> > > lockdep_assert_irqs_disabled();
> > > +
> > > + /*
> > > + * #DB is special and needs to be handled outside of the intrumentation_begin()/end().
> > > + * Otherwise the #VC handler could be raised recursivly.
> > > + */
> > > + if (error_code == SVM_EXIT_EXCP_BASE + X86_TRAP_DB) {
> > > + vc_handle_trap_db(regs);
> > > + return;
> > > + }
> > > +
> > > instrumentation_begin();
> >
> > Wait what?! That makes no sense what so ever.
>
> Then my understanding of intrumentation_begin/end() is wrong, I thought
> that the kernel will forbid setting breakpoints before
> instrumentation_begin(), which is necessary here because a break-point
> in the #VC handler might cause recursive #VC-exceptions when #DB is
> intercepted.
> Maybe you can elaborate on why this makes no sense?

Kernel avoids breakpoints in any noinstr text, irrespective of
instrumentation_begin().

instrumentation_begin() merely allows one to call !noinstr functions.