From mboxrd@z Thu Jan  1 00:00:00 1970
From: Peter Zijlstra <peterz@infradead.org>
Subject: Re: Should SEV-ES #VC use IST? (Re: [PATCH] Allow RDTSC and RDTSCP
 from userspace)
Date: Tue, 23 Jun 2020 14:52:01 +0200
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To: Joerg Roedel <jroedel@suse.de>
Cc: Juergen Gross <JGross@suse.com>, Tom Lendacky <Thomas.Lendacky@amd.com>, Thomas Hellstrom <thellstrom@vmware.com>, X86 ML <x86@kernel.org>, Mike Stunes <mstunes@vmware.com>, Kees Cook <keescook@chromium.org>, kvm list <kvm@vger.kernel.org>, Andrew Cooper <andrew.cooper3@citrix.com>, Joerg Roedel <joro@8bytes.org>, Dave Hansen <dave.hansen@linux.intel.com>, LKML <linux-kernel@vger.kernel.org>, Sean Christopherson <sean.j.christopherson@intel.com>, Linux Virtualization <virtualization@lists.linux-foundation.org>, Dave Hansen <dave.hansen@intel.com>, Andy Lutomirski <luto@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>, Dan Williams <dan.j.williams@intel.com>, Jiri Slaby <jslaby@suse.cz>
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On Tue, Jun 23, 2020 at 02:04:33PM +0200, Joerg Roedel wrote:
> On Tue, Jun 23, 2020 at 01:48:18PM +0200, Peter Zijlstra wrote:
> > On Tue, Jun 23, 2020 at 01:30:07PM +0200, Joerg Roedel wrote:
> 
> > 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..).
> 
> No, the recursion check is fine, because overwriting an already used IST
> stack doesn't matter (as long as it can be detected) if we are going to
> panic anyway. It doesn't matter because the kernel will not leave the
> currently running handler anymore.

You only have that guarantee when any SNP #VC from kernel is an
automatic panic. But in that case, what's the point of having the
recursion count?

> > I'll keep repeating this, x86_64 exceptions are a trainwreck, and IST in
> > specific is utter crap.
> 
> I agree, but don't forget the most prominent underlying reason for IST:
> The SYSCALL gap. If SYSCALL would switch stacks most of those issues
> would not exist. IST would still be needed because there are no task
> gates in x86-64, but still...

We could all go back to int80 ;-) /me runs like heck