x86/spec-ctrl: Rework conditional safety for SPEC_CTRL_ENTRY_*

x86/spec-ctrl: Rework conditional safety for SPEC_CTRL_ENTRY_*

Right now, we have a mix of safety strategies in different blocks, making the
logic fragile and hard to follow.

Start addressing this by having a safety LFENCE at the end of the blocks,
which can be patched out if other safety criteria are met.  This will allow us
to simplify the sub-blocks.  For SPEC_CTRL_ENTRY_FROM_IST, simply leave an
LFENCE unconditionally at the end; the IST path is not a fast-path by any
stretch of the imagination.

For SPEC_CTRL_ENTRY_FROM_INTR, the existing description was incorrect.  The
IRET #GP path is non-fatal but can occur with the guest's choice of
MSR_SPEC_CTRL.  It is safe to skip the flush/barrier-like protections when
interrupting Xen, but we must run DO_SPEC_CTRL_ENTRY irrespective.

This will skip RSB stuffing which was previously unconditional even when
interrupting Xen.

AFAICT, this is a missing cleanup from commit 3fffaf9c13e9 ("x86/entry: Avoid
using alternatives in NMI/#MC paths") where we split the IST entry path out of
the main INTR entry path.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
(cherry picked from commit 94896de1a98c4289fe6fef9e16ef99fc6ef2efc4)