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

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Date:   Wed, 15 Jul 2020 11:51:36 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Joerg Roedel <jroedel@...e.de>
Cc:     Joerg Roedel <joro@...tes.org>, x86@...nel.org, hpa@...or.com,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Jiri Slaby <jslaby@...e.cz>,
        Dan Williams <dan.j.williams@...el.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Juergen Gross <jgross@...e.com>,
        Kees Cook <keescook@...omium.org>,
        David Rientjes <rientjes@...gle.com>,
        Cfir Cohen <cfir@...gle.com>,
        Erdem Aktas <erdemaktas@...gle.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Mike Stunes <mstunes@...are.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Martin Radev <martin.b.radev@...il.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v4 63/75] x86/sev-es: Handle #DB Events

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.