KVM: x86/mmu: Defer TLB flush to caller when freeing TDP MMU shadow pages

Defer TLB flushes to the caller when freeing TDP MMU shadow pages instead
of immediately flushing.  Because the shadow pages are freed in an RCU
callback, so long as at least one CPU holds RCU, all CPUs are protected.
For vCPUs running in the guest, i.e. consuming TLB entries, KVM only
needs to ensure the caller services the pending TLB flush before dropping
its RCU protections.  I.e. use the caller's RCU as a proxy for all vCPUs
running in the guest.

Deferring the flushes allows batching flushes, e.g. when installing a
1gb hugepage and zapping a pile of SPs.  And when zapping an entire root,
deferring flushes allows skipping the flush entirely (because flushes are
not needed in that case).

Avoiding flushes when zapping an entire root is especially important as
synchronizing with other CPUs via IPI after zapping every shadow page can
cause significant performance issues for large VMs.  The issue is
exacerbated by KVM zapping entire top-level entries without dropping
RCU protection, which can lead to RCU stalls even when zapping roots
backing relatively "small" amounts of guest memory, e.g. 2tb.  Removing
the IPI bottleneck largely mitigates the RCU issues, though it's likely
still a problem for 5-level paging.  A future patch will further address
the problem by zapping roots in multiple passes to avoid holding RCU for
an extended duration.

Reviewed-by: Ben Gardon <bgardon@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220226001546.360188-20-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>