From mboxrd@z Thu Jan  1 00:00:00 1970
From: Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH v4 63/75] x86/sev-es: Handle #DB Events
Date: Wed, 15 Jul 2020 11:51:36 +0200
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To: Joerg Roedel <jroedel@suse.de>
Cc: Juergen Gross <jgross@suse.com>, Tom Lendacky <thomas.lendacky@amd.com>, Dave Hansen <dave.hansen@linux.intel.com>, Mike Stunes <mstunes@vmware.com>, Kees Cook <keescook@chromium.org>, kvm@vger.kernel.org, Cfir Cohen <cfir@google.com>, Joerg Roedel <joro@8bytes.org>, x86@kernel.org, linux-kernel@vger.kernel.org, Sean Christopherson <sean.j.christopherson@intel.com>, virtualization@lists.linux-foundation.org, Martin Radev <martin.b.radev@gmail.com>, Masami Hiramatsu <mhiramat@kernel.org>, Andy Lutomirski <luto@kernel.org>, hpa@zytor.com, Erdem Aktas <erdemaktas@google.com>, David Rientjes <rientjes@google.com>, Dan Williams <dan.j.williams@intel.com>, Jiri Slaby <jslaby@suse.cz>
List-Id: virtualization@lists.linuxfoundation.org

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.