x86/hap: improve hypervisor assisted guest TLB flush

x86/hap: improve hypervisor assisted guest TLB flush

The current implementation of the hypervisor assisted flush for HAP is
extremely inefficient.

First of all there's no need to call paging_update_cr3, as the only
relevant part of that function when doing a flush is the ASID vCPU
flush, so just call that function directly.

Since hvm_asid_flush_vcpu is protected against concurrent callers by
using atomic operations there's no need anymore to pause the affected
vCPUs.

Finally the global TLB flush performed by flush_tlb_mask is also not
necessary, since we only want to flush the guest TLB state it's enough
to trigger a vmexit on the pCPUs currently holding any vCPU state, as
such vmexit will already perform an ASID/VPID update, and thus clear
the guest TLB.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Wei Liu <wl@xen.org>
Reviewed-by: Jan Beulich <jbeulich@suse.com>