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Date:   Tue, 23 Jun 2020 13:48:18 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Joerg Roedel <jroedel@suse.de>
Cc:     Andy Lutomirski <luto@kernel.org>, Joerg Roedel <joro@8bytes.org>,
        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>,
        Thomas Hellstrom <thellstrom@vmware.com>,
        Linux Virtualization <virtualization@lists.linux-foundation.org>,
        X86 ML <x86@kernel.org>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Andrew Cooper <andrew.cooper3@citrix.com>
Subject: Re: Should SEV-ES #VC use IST? (Re: [PATCH] Allow RDTSC and RDTSCP
 from userspace)
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On Tue, Jun 23, 2020 at 01:30:07PM +0200, Joerg Roedel wrote:
> Note that this is an issue only with secure nested paging (SNP), which
> is not enabled yet with this patch-set. When it gets enabled a stack
> recursion check in the #VC handler is needed which panics the VM. That
> also fixes the #VC-in-early-NMI problem.

But you cannot do a recursion check in #VC, because the NMI can happen
on the first instruction of #VC, before we can increment our counter,
and then the #VC can happen on NMI because the IST stack is a goner, and
we're fscked again (or on a per-cpu variable we touch in our elaborate
NMI setup, etc..).

There is no way I can see SNP-#VC 'work'. The best I can come up with is
'mostly', but do you like your bridges/dikes/etc.. to be mostly ok? Or
do you want a guarantee they'll actually work?

I'll keep repeating this, x86_64 exceptions are a trainwreck, and IST in
specific is utter crap.