x86/xpti: Don't leak TSS-adjacent percpu data via Meltdown

x86/xpti: Don't leak TSS-adjacent percpu data via Meltdown

The XPTI work restricted the visibility of most of memory, but missed a few
aspects when it came to the TSS.

Given that the TSS is just an object in percpu data, the 4k mapping for it
created in setup_cpu_root_pgt() maps adjacent percpu data, making it all
leakable via Meltdown, even when XPTI is in use.

Furthermore, no care is taken to check that the TSS doesn't cross a page
boundary.  As it turns out, struct tss_struct is aligned on its size which
does prevent it straddling a page boundary.

Rework the TSS types while making this change.  Rename tss_struct to tss64, to
mirror the existing tss32 structure we have in HVM's Tast Switch logic.  Drop
tss64's alignment and __cacheline_filler[] field.

Introduce tss_page which contains a single tss64 and keeps the rest of the
page clear, so no adjacent data can be leaked.  Move the definition from
setup.c to traps.c, which is a more appropriate place for it to live.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>