linux-kernel - Re: [PATCH 62/62] x86/sev-es: Add NMI state tracking

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Message-ID: <20200212135645.GK20066@8bytes.org>
Date:   Wed, 12 Feb 2020 14:56:45 +0100
From:   Joerg Roedel <joro@...tes.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     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>,
        Joerg Roedel <jroedel@...e.de>
Subject: Re: [PATCH 62/62] x86/sev-es: Add NMI state tracking

On Tue, Feb 11, 2020 at 02:50:29PM -0800, Andy Lutomirski wrote:
> On Tue, Feb 11, 2020 at 5:53 AM Joerg Roedel <joro@...tes.org> wrote:
> This patch is overcomplicated IMO.  Just do the magic incantation in C
> from do_nmi or from here:
> 
>         /*
>          * For ease of testing, unmask NMIs right away.  Disabled by
>          * default because IRET is very expensive.
> 
> If you do the latter, you'll need to handle the case where the NMI
> came from user mode.
> 
> The ideal solution is do_nmi, I think.
> 
> if (static_cpu_has(X86_BUG_AMD_FORGOT_ABOUT_NMI))
>   sev_es_unmask_nmi();
> 
> Feel free to use X86_FEATURE_SEV_ES instead :)

Yeah, I also had that implemented once, but then changed it because I
thought that nested NMIs do not necessarily call into do_nmi(), which
would cause NMIs to stay blocked forever. I need to read through the NMI
entry code again to check if that can really happen.

Regards,

	Joerg