KVM: x86/mmu: Batch TLB flushes when zapping collapsible TDP MMU SPTEs

KVM: x86/mmu: Batch TLB flushes when zapping collapsible TDP MMU SPTEs

Set SPTEs directly to SHADOW_NONPRESENT_VALUE and batch up TLB flushes
when zapping collapsible SPTEs, rather than freezing them first.

Freezing the SPTE first is not required. It is fine for another thread
holding mmu_lock for read to immediately install a present entry before
TLBs are flushed because the underlying mapping is not changing. vCPUs
that translate through the stale 4K mappings or a new huge page mapping
will still observe the same GPA->HPA translations.

KVM must only flush TLBs before dropping RCU (to avoid use-after-free of
the zapped page tables) and before dropping mmu_lock (to synchronize
with mmu_notifiers invalidating mappings).

In VMs backed with 2MiB pages, batching TLB flushes improves the time it
takes to zap collapsible SPTEs to disable dirty logging:

 $ ./dirty_log_perf_test -s anonymous_hugetlb_2mb -v 64 -e -b 4g

 Before: Disabling dirty logging time: 14.334453428s (131072 flushes)
 After:  Disabling dirty logging time: 4.794969689s  (76 flushes)

Skipping freezing SPTEs also avoids stalling vCPU threads on the frozen
SPTE for the time it takes to perform a remote TLB flush. vCPUs faulting
on the zapped mapping can now immediately install a new huge mapping and
proceed with guest execution.

Signed-off-by: David Matlack <dmatlack@google.com>
Link: https://lore.kernel.org/r/20240823235648.3236880-3-dmatlack@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>