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Date:   Wed, 15 Jul 2020 11:24:56 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Joerg Roedel <joro@8bytes.org>
Cc:     x86@kernel.org, Joerg Roedel <jroedel@suse.de>, 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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On Tue, Jul 14, 2020 at 02:08:02PM +0200, Joerg Roedel wrote:
> The #VC entry code now tries to pretend that the #VC handler does not
> use an IST stack by switching to the task stack if entered from
> user-mode or the SYSCALL entry path. When it is entered from
> kernel-mode it is doing its best to switch back to the interrupted
> stack. This is only possible if it is entered from a known and safe
> kernel stack (e.g. not the entry stack). If the previous stack is not
> safe to use the #VC handler switches to a fall-back stack and calls a
> special handler function which, as of now, just panics the system. For
> now this is safe as #VC exceptions only happen at know places which
> use a safe stack.
> 
> The use of the fall-back stack is necessary so that the special
> handler function can safely raise nested #VC exceptions, for
> example to print a panic message.

Can we get some more words -- preferably in actual code comments, on
when exactly #VC happens?

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.