x86: suppress BTI mitigations around S3 suspend/resume

x86: suppress BTI mitigations around S3 suspend/resume

NMI and #MC can occur at any time after S3 resume, yet the MSR_SPEC_CTRL
may become available only once we're reloaded microcode. Make
SPEC_CTRL_ENTRY_FROM_INTR_IST and DO_SPEC_CTRL_EXIT_TO_XEN no-ops for
the critical period of time.

Also set the MSR back to its intended value.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
x86: Use spec_ctrl_{enter,exit}_idle() in the S3/S5 path

The main purpose of this patch is to avoid opencoding the recovery logic at
the end, but also has the positive side effect of relaxing the SPEC_CTRL
mitigations when working to shut the final CPU down.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
master commit: 710a8ebf2bc111a34bba04d1c85b6d07ed3d9389
master date: 2018-04-16 14:09:55 +0200
master commit: ef3ab46493f650b7e5cca2b2578a99ca0cbff195
master date: 2018-04-19 10:55:59 +0100