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From:   Andy Lutomirski <luto@kernel.org>
Date:   Sat, 25 Apr 2020 15:10:29 -0700
Message-ID: <CALCETrW2Y6UFC=zvGbXEYqpsDyBh0DSEM4NQ+L=_pp4aOd6Fuw@mail.gmail.com>
Subject: Re: [PATCH] Allow RDTSC and RDTSCP from userspace
To:     Joerg Roedel <joro@8bytes.org>
Cc:     Andy Lutomirski <luto@kernel.org>, Joerg Roedel <jroedel@suse.de>,
        Dave Hansen <dave.hansen@intel.com>,
        Tom Lendacky <Thomas.Lendacky@amd.com>,
        Mike Stunes <mstunes@vmware.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Juergen Gross <JGross@suse.com>,
        Jiri Slaby <jslaby@suse.cz>, Kees Cook <keescook@chromium.org>,
        kvm list <kvm@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Hellstrom <thellstrom@vmware.com>,
        Linux Virtualization <virtualization@lists.linux-foundation.org>,
        X86 ML <x86@kernel.org>,
        Sean Christopherson <sean.j.christopherson@intel.com>
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On Sat, Apr 25, 2020 at 1:23 PM Joerg Roedel <joro@8bytes.org> wrote:
>
> On Sat, Apr 25, 2020 at 12:47:31PM -0700, Andy Lutomirski wrote:
> > I assume the race you mean is:
> >
> > #VC
> > Immediate NMI before IST gets shifted
> > #VC
> >
> > Kaboom.
> >
> > How are you dealing with this?  Ultimately, I think that NMI will need
> > to turn off IST before engaging in any funny business. Let me ponder
> > this a bit.
>
> Right, I dealt with that by unconditionally shifting/unshifting the #VC IST entry
> in do_nmi() (thanks to Davin Kaplan for the idea). It might cause
> one of the IST stacks to be unused during nesting, but that is fine. The
> stack memory for #VC is only allocated when SEV-ES is active (in an
> SEV-ES VM).

Blech.  It probably works, but still, yuck.  It's a bit sad that we
seem to be growing more and more poorly designed happens-anywhere
exception types at an alarming rate.  We seem to have #NM, #MC, #VC,
#HV, and #DB.  This doesn't really scale.

--Andy