x86/ctxt-switch: Document and improve GDT handling

x86/ctxt-switch: Document and improve GDT handling

Calling virt_to_mfn() in the context switch path is a lot
of wasted cycles for a result which is constant after boot.

Begin by documenting how Xen handles the GDTs across context switch.

The loop in write_full_gdt_ptes() is unnecessary, because
NR_RESERVED_GDT_PAGES is 1.  Dropping it makes the code substantially
more clear, and with it dropped, write_full_gdt_ptes() becomes more
obviously a poor name, so rename it to update_xen_slot_in_full_gdt().

Furthermore, load_full_gdt() is completely independent of the current
CPU, and load_default_gdt() only needs the current CPU's regular
GDT.  (This is a change in behaviour, as previously it may have used the
compat GDT, but either will do.)

Add two extra per-cpu variables which cache the L1e for the regular and compat
GDT, calculated in cpu_smpboot_alloc()/trap_init() as appropriate, so
update_xen_slot_in_full_gdt() doesn't need to waste time performing the same
calculation on every context switch.

One performance scenario of Jüergen's (time to build the hypervisor on
an 8 CPU system, with two single-vCPU MiniOS VMs constantly interrupting
dom0 with events) shows the following, average over 5 measurements:

            elapsed  user   system
  Unpatched  66.51  232.93  109.21
  Patched    57.00  225.47  105.47

which is a substantial improvement.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Tested-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>