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[Xen-devel] [RESEND] Spectre-v2 (IBPB/IBRS) and SSBD fixes for 4.4.y

Hi Greg,

This patch series is a backport of the Spectre-v2 fixes (IBPB/IBRS)
and patches for the Speculative Store Bypass vulnerability to 4.4.y
(they apply cleanly on top of 4.4.140).

I used 4.9.y as my reference when backporting to 4.4.y (as I thought
that would minimize the amount of fixing up necessary). Unfortunately
I had to skip the KVM fixes for these vulnerabilities, as the KVM
codebase is drastically different in 4.4 as compared to 4.9. (I tried
my best to backport them initially, but wasn't confident that they
were correct, so I decided to drop them from this series).

You'll notice that the initial few patches in this series include
cleanups etc., that are non-critical to IBPB/IBRS/SSBD. Most of these
patches are aimed at getting the cpufeature.h vs cpufeatures.h split
into 4.4, since a lot of the subsequent patches update these headers.
On my first attempt to backport these patches to 4.4.y, I had actually
tried to do all the updates on the cpufeature.h file itself, but it
started getting very cumbersome, so I resorted to backporting the
cpufeature.h vs cpufeatures.h split and their dependencies as well. I
think apart from these initial patches, the rest of the patchset
doesn't have all that much noise. 

This patchset has been tested on both Intel and AMD machines (Intel
Xeon CPU E5-2660 v4 and AMD EPYC 7281 16-Core Processor, respectively)
with updated microcode. All the patch backports have been
independently reviewed by Matt Helsley, Alexey Makhalov and Bo Gan.

I would appreciate if you could kindly consider these patches for
review and inclusion in a future 4.4.y release.

Thank you very much!

VMware Photon OS

P.S. This patchset is also available in the following repo if anyone
     is interested in giving it a try:

https://github.com/srivatsabhat/linux-stable spectre-v2-fixes-nokvm-4.4.140

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