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Date:   Wed, 15 Jul 2020 11:55:56 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Joerg Roedel <jroedel@suse.de>
Cc:     Joerg Roedel <joro@8bytes.org>, x86@kernel.org, hpa@zytor.com,
        Andy Lutomirski <luto@kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Jiri Slaby <jslaby@suse.cz>,
        Dan Williams <dan.j.williams@intel.com>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        Juergen Gross <jgross@suse.com>,
        Kees Cook <keescook@chromium.org>,
        David Rientjes <rientjes@google.com>,
        Cfir Cohen <cfir@google.com>,
        Erdem Aktas <erdemaktas@google.com>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Mike Stunes <mstunes@vmware.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Martin Radev <martin.b.radev@gmail.com>,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        virtualization@lists.linux-foundation.org
Subject: Re: [PATCH v4 00/75] x86: SEV-ES Guest Support
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 <20200715092456.GE10769@hirez.programming.kicks-ass.net>
 <20200715093426.GK16200@suse.de>
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On Wed, Jul 15, 2020 at 11:34:26AM +0200, Joerg Roedel wrote:
> On Wed, Jul 15, 2020 at 11:24:56AM +0200, Peter Zijlstra wrote:
> > Can we get some more words -- preferably in actual code comments, on
> > when exactly #VC happens?
> 
> Sure, will add this as a comment before the actual runtime VC handler.

Thanks!

> > Because the only thing I remember is that #VC could happen on any memop,
> > but I also have vague memories of that being a later extention.
> 
> Currently it is only raised when something happens that the hypervisor
> intercepts, for example on a couple of instructions like CPUID,
> RD/WRMSR, ..., or on MMIO/IOIO accesses.
> 
> With Secure Nested Paging (SNP), which needs additional enablement, a #VC can
> happen on any memory access. I wrote the IST handling entry code for #VC
> with that in mind, but do not actually enable it. This is the reason why
> the #VC handler just panics the system when it ends up on the fall-back
> (VC2) stack, with SNP enabled it needs to handle the SNP exit-codes in
> that path.

And recursive #VC was instant death, right? Because there's no way to
avoid IST stack corruption in that case.