x86/tlb: fix assisted flush usage

x86/tlb: fix assisted flush usage

Commit e9aca9470ed86 introduced a regression when avoiding sending
IPIs for certain flush operations. Xen page fault handler
(spurious_page_fault) relies on blocking interrupts in order to
prevent handling TLB flush IPIs and thus preventing other CPUs from
removing page tables pages. Switching to assisted flushing avoided such
IPIs, and thus can result in pages belonging to the page tables being
removed (and possibly re-used) while __page_fault_type is being
executed.

Force some of the TLB flushes to use IPIs, thus avoiding the assisted
TLB flush. Those selected flushes are the page type change (when
switching from a page table type to a different one, ie: a page that
has been removed as a page table) and page allocation. This sadly has
a negative performance impact on the pvshim, as less assisted flushes
can be used. Note the flush in grant-table code is also switched to
use an IPI even when not strictly needed. This is done so that a
common arch_flush_tlb_mask can be introduced and always used in common
code.

Introduce a new flag (FLUSH_FORCE_IPI) and helper to force a TLB flush
using an IPI (x86 only). Note that the flag is only meaningfully defined
when the hypervisor supports PV or shadow paging mode, as otherwise
hardware assisted paging domains are in charge of their page tables and
won't share page tables with Xen, thus not influencing the result of
page walks performed by the spurious fault handler.

Just passing this new flag when calling flush_area_mask prevents the
usage of the assisted flush without any other side effects.

Note the flag is not defined on Arm.

Fixes: e9aca9470ed86 ('x86/tlb: use Xen L0 assisted TLB flush when available')
Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Julien Grall <jgrall@amazon.com>
Release-acked-by: Paul Durrant <paul@xen.org>