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Date:   Tue, 23 Jun 2020 18:02:15 +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 05:38:55PM +0200, Joerg Roedel wrote:
> On Tue, Jun 23, 2020 at 05:23:26PM +0200, Peter Zijlstra wrote:

> > Reliability of that depends on the unwinder, I wouldn't want the guess
> > uwinder to OOPS me by accident.
> 
> It doesn't use the full unwinder, it just assumes that there is a
> pt_regs struct at the top of every kernel stack and walks through them
> until SP points to a user-space stack.
> 
> As long as the assumption that there is a pt_regs struct on top of every
> stack holds, this should be safe. The assumption might be wrong when an
> exception happens during SYSCALL/SYSENTER entry, when the return frame
> is not written by hardware.

The IRQ and SoftIRQ stacks don't have that I think. Only the task and
exception stacks.