linux-kernel - Re: [PATCH v3 47/75] x86/sev-es: Add Runtime #VC Exception Handler

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Message-ID: <20200611114831.GA11924@8bytes.org>
Date:   Thu, 11 Jun 2020 13:48:31 +0200
From:   Joerg Roedel <joro@...tes.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     x86@...nel.org, hpa@...or.com, Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Hellstrom <thellstrom@...are.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>,
        Joerg Roedel <jroedel@...e.de>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v3 47/75] x86/sev-es: Add Runtime #VC Exception Handler

On Sat, May 23, 2020 at 09:59:24AM +0200, Borislav Petkov wrote:
> On Tue, Apr 28, 2020 at 05:16:57PM +0200, Joerg Roedel wrote:
> > +	/*
> > +	 * Mark the per-cpu GHCBs as in-use to detect nested #VC exceptions.
> > +	 * There is no need for it to be atomic, because nothing is written to
> > +	 * the GHCB between the read and the write of ghcb_active. So it is safe
> > +	 * to use it when a nested #VC exception happens before the write.
> > +	 */
> 
> Looks liks that is that text... support for nested #VC exceptions.
> I'm sure this has come up already but why do we even want to support
> nested #VCs? IOW, can we do without them first or are they absolutely
> necessary?
> 
> I'm guessing VC exceptions inside the VC handler but what are the
> sensible use cases?

The most important use-case is #VC->NMI->#VC. When an NMI hits while the
#VC handler uses the GHCB and the NMI handler causes another #VC, then
the contents of the GHCB needs to be backed up, so that it doesn't
destroy the GHCB contents of the first #VC handling path.


Regards,

	Joerg