KVM: x86/mmu: Recover TDP MMU NX huge pages using MMU read lock

KVM: x86/mmu: Recover TDP MMU NX huge pages using MMU read lock

Use MMU read lock to recover TDP MMU NX huge pages.  To prevent
concurrent modification of the list of potential huge pages, iterate over
the list under tdp_mmu_pages_lock protection and unaccount the page
before dropping the lock.

Zapping under MMU read lock unblocks vCPUs which are waiting for MMU
read lock, which solves a guest jitter issue on Windows VMs which were
observing an increase in network latency.

Do not zap an SPTE if:
- The SPTE is a root page.
- The SPTE does not point at the SP's page table.

If the SPTE does not point at the SP's page table, then something else
has change the SPTE, so KVM cannot safely zap it.

Warn if zapping SPTE fails and current SPTE is still pointing to same
page table, as it should be impossible for the CMPXCHG to fail due to all
other write scenarios being mutually exclusive.

There is always a race between dirty logging, vCPU faults, and NX huge
page recovery for backing a gfn by an NX huge page or an executable
small page.  Unaccounting sooner during the list traversal increases the
window of that race, but functionally, it is okay.  Accounting doesn't
protect against iTLB multi-hit bug, it is there purely to prevent KVM
from bouncing a gfn between two page sizes. The only  downside is that a
vCPU will end up doing more work in tearing down all  the child SPTEs.
This should be a very rare race.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Vipin Sharma <vipinsh@google.com>
Co-developed-by: James Houghton <jthoughton@google.com>
Signed-off-by: James Houghton <jthoughton@google.com>
Link: https://lore.kernel.org/r/20250707224720.4016504-4-jthoughton@google.com
[sean: clean up kvm_mmu_sp_dirty_logging_enabled() and the changelog]
Signed-off-by: Sean Christopherson <seanjc@google.com>