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Date:   Tue, 23 Jun 2020 13:50:14 +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 01:43:24PM +0200, Joerg Roedel wrote:
> On Tue, Jun 23, 2020 at 01:14:43PM +0200, Peter Zijlstra wrote:
> > On Tue, Jun 23, 2020 at 01:11:07PM +0200, Joerg Roedel wrote:
> 
> > > 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.
> 
> As I wrote in the other mail, this can only happen when SNP gets enabled
> (which is follow-on work to this) and is handled by a stack recursion
> check in the #VC handler.
> 
> The reason I mentioned the #VC-anywhere case is to make it more clear
> why #VC needs an IST handler.

If SNP is the sole reason #VC needs to be IST, then I'd strongly urge
you to only make it IST if/when you try and make SNP happen, not before.