linux-kernel - Re: [PATCH 70/70] x86/sev-es: Add NMI state tracking

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Date:   Fri, 20 Mar 2020 20:48:49 +0100
From:   Joerg Roedel <joro@...tes.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Joerg Roedel <jroedel@...e.de>, X86 ML <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        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>,
        LKML <linux-kernel@...r.kernel.org>,
        kvm list <kvm@...r.kernel.org>,
        Linux Virtualization <virtualization@...ts.linux-foundation.org>
Subject: Re: [PATCH 70/70] x86/sev-es: Add NMI state tracking

On Thu, Mar 19, 2020 at 02:27:49PM -0700, Andy Lutomirski wrote:
> AIUI the shift-ist stuff is because we aren't very good about the way
> that we handle tracing right now, and that can cause a limited degree
> of recursion.  #DB uses IST for historical reasons that don't
> necessarily make sense.  Right now, we need it for only one reason:
> the MOV SS issue.  IIRC this isn't actually triggerable without
> debugging enabled -- MOV SS with no breakpoint but TF on doesn't seem
> to malfunction quite as badly.

I had a look at the shift_ist stuff today and it looks like a good
solution to the #VC nesting problem when it is turned into a #VC
handler. The devil is in the details, of course, as 3 or 4 stacks for
the #VC handler (per cpu) should only be allocated when actually running
in an SEV-ES guest. Let's see how this works out in practice.

Regards,

	Joerg