Re: [PATCH] selftests/x86: Fix sysret_rip assertion failure on FRED systems

[Andrew Cooper <andrew.cooper3@citrix.com>]

> First, CPUID doesn't tell you if FRED is in use. Is it even on by
> default yet? There might not be a better way to do this than checking
> CPUID, but checking CPUID is imprecise at best.

A reliable way to distinguish IDT and FRED mode is to:

1) Load $3 into %fs (x86_64) or %gs (i386) (i.e. whichever isn't thread
local stoage)
2) execute a breakpoint, ignore the signal
3) Look to see whether %fs/%gs holds 3 or 0

IRET has a fun behaviour where it zeroes NULL selectors even if they had
a non-zero RPL.

ERETU doesn't do this; Andy Luto and I asked for this minor information
leak to be removed, and Intel agreed as it served no purpose anyone
could identify.

As a consequence, you can use it to determine whether the kernel used
IRET or ERET to return back to userspace.

~Andrew