linux-kernel - Re: [PATCH] Allow RDTSC and RDTSCP from userspace

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Date:   Sat, 25 Apr 2020 22:23:16 +0200
From:   Joerg Roedel <joro@...tes.org>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Andy Lutomirski <luto@...nel.org>, Joerg Roedel <jroedel@...e.de>,
        Dave Hansen <dave.hansen@...el.com>,
        Tom Lendacky <Thomas.Lendacky@....com>,
        Mike Stunes <mstunes@...are.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>, Juergen Gross <JGross@...e.com>,
        Jiri Slaby <jslaby@...e.cz>, Kees Cook <keescook@...omium.org>,
        kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Hellstrom <thellstrom@...are.com>,
        Linux Virtualization <virtualization@...ts.linux-foundation.org>,
        X86 ML <x86@...nel.org>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Subject: Re: [PATCH] Allow RDTSC and RDTSCP from userspace

On Sat, Apr 25, 2020 at 12:47:31PM -0700, Andy Lutomirski wrote:
> I assume the race you mean is:
> 
> #VC
> Immediate NMI before IST gets shifted
> #VC
> 
> Kaboom.
> 
> How are you dealing with this?  Ultimately, I think that NMI will need
> to turn off IST before engaging in any funny business. Let me ponder
> this a bit.

Right, I dealt with that by unconditionally shifting/unshifting the #VC IST entry
in do_nmi() (thanks to Davin Kaplan for the idea). It might cause
one of the IST stacks to be unused during nesting, but that is fine. The
stack memory for #VC is only allocated when SEV-ES is active (in an
SEV-ES VM).

Regards,

	Joerg