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

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Date:   Wed, 15 Jul 2020 12:13:10 +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 12:08:08PM +0200, Joerg Roedel wrote:
> On Wed, Jul 15, 2020 at 11:51:36AM +0200, Peter Zijlstra wrote:
> > On Wed, Jul 15, 2020 at 11:13:37AM +0200, Joerg Roedel wrote:
> > > 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.
> 
> Right, but the handler calls into various other functions. I actually
> started to annotate them all with noinstr, but that was a can of worms
> when calling into generic kernel functions. And the only problem with
> intrumentation in the #VC handler is the #VC-for-#DB exit-code, so I
> decided to only handle this one with instrumentation forbidden and allow
> it for the rest of the handler.

OK, then maybe change the comment to something like:

 /*
  * Handle #DB before calling any !noinstr code to avoid
  * recursive #DB.
  */

?