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Date:   Tue, 23 Jun 2020 13:14:43 +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 01:11:07PM +0200, Joerg Roedel wrote:
> Hi Peter,
> 
> On Tue, Jun 23, 2020 at 12:45:59PM +0200, Peter Zijlstra wrote:
> > On Tue, Jun 23, 2020 at 11:45:19AM +0200, Joerg Roedel wrote:
> > > Or maybe you have a better idea how to implement this, so I'd like to
> > > hear your opinion first before I spend too many days implementing
> > > something.
> > 
> > OK, excuse my ignorance, but I'm not seeing how that IST shifting
> > nonsense would've helped in the first place.
> > 
> > If I understand correctly the problem is:
> > 
> > 	<#VC>
> > 	  shift IST
> > 	  <NMI>
> > 	    ... does stuff
> > 	    <#VC> # again, safe because the shift
> > 
> > But what happens if you get the NMI before your IST adjustment?
> 
> The v3 patchset implements an unconditional shift of the #VC IST entry
> in the NMI handler, before it can trigger a #VC exception.

Going by that other thread -- where you said that any memory access can
trigger a #VC, there just isn't such a guarantee.

> > Either way around we get to fix this up in NMI (and any other IST
> > exception that can happen while in #VC, hello #MC). And more complexity
> > there is the very last thing we need :-(
> 
> Yes, in whatever way this gets implemented, it needs some fixup in the
> NMI handler. But that can happen in C code, so it does not make the
> assembly more complex, at least.
> 
> > There's no way you can fix up the IDT without getting an NMI first.
> 
> Not sure what you mean by this.

I was talking about the case where #VC would try and fix up its own IST.