linux-kernel - Linux FSGSBASE testing

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Date:   Sat, 20 Jun 2020 08:59:57 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Stas Sergeev <stsp@...t.ru>
Subject: Linux FSGSBASE testing

Hi Stas-

FSGSBASE support is queued up for Linux 5.9.  Since you're one of the
more exotic users of segmentation on Linux, is there any chance you
could test it?  The code is here:

https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/log/?h=x86/fsgsbase

There are two interesting cases to test:

1. FSGSBASE on.  This is the default if you boot this kernel on Ivy
Bridge or newer hardware.

2. FSGSBASE off on a patched kernel.  Boot the same kernel as in #1
but either pass nofsgsbase on the kernel command line or use pre-Ivy
Bridge hardware.  You will *

You can tell you have FSGSBASE enabled for test #1 by running
tools/testing/selftests/x86/fsgsbase_64 -- the first line of output
will be :FSGSBASE instructions are enabled".  You can build it by
cd-ing to tools/testing/selftests/x86 and running make.

If anything is broken for you, I'd like to know before this makes it
into a released kernel!

Thanks,
Andy