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Date:   Tue, 23 Jun 2020 17:52:04 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Andrew Cooper <andrew.cooper3@citrix.com>
Cc:     Joerg Roedel <jroedel@suse.de>, 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>
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 04:39:26PM +0100, Andrew Cooper wrote:
> On 23/06/2020 16:23, Peter Zijlstra wrote:
> > On Tue, Jun 23, 2020 at 04:59:14PM +0200, Joerg Roedel wrote:

> >> Yes, this is a start, it doesn't cover the case where the NMI stack is
> >> in-between, so I think you need to walk down regs->sp too.
> > That shouldn't be possible with the current code, I think.
> 
> NMI; #MC; Anything which IRET but isn't fatal - #DB, or #BP from
> patching, #GP from *_safe(), etc; NMI
> 
> Sure its a corner case, but did you hear that IST is evil?

Isn't current #MC unconditionally fatal from kernel? But yes, I was
sorta aware people want that changed.

And yes, NMI can recurse, mostly on #BP and #PF. Like I wrote, its
broken vs #MC.

But Joerg was talking about IST recursion with NMI in the middle,
something like: #DB, NMI, #DB, and not already being fatal. This one in
particular is ruled out by #DB itself clearing DR7 (but NMI would also
do that).

> P.S. did you also hear that with Rowhammer, userspace has a nonzero
> quantity of control over generating #MC, depending on how ECC is
> configured on the platform.

Yes, excellent stuff.