[RFC PATCH 0/2] KVM: arm64: nv: Skip empty spans when unmapping a shadow S2

[Haseeb Ashraf <haseebashraf091@gmail.com>]

Emulating TLBI VMALLS12E1IS, emulating TLBI ALLE1IS and recycling a
shadow stage-2 all unmap the whole nested IPA space in chunks of the
largest block size, even when the table is empty. With a 48-bit PARange
and 4kB pages that is 262144 unmap calls per shadow MMU, each ending in a
range TLBI when FEAT_TLBIRANGE and FWB are available.

Measured cost: one full-range unmap of a shadow MMU takes 0.87-1.06 s on
AmpereOne (Xen on KVM, 4kB pages, function_graph). On Graviton4
(Xen-on-KVM, 48-bit IPA) a single VMALLS12E1IS trap took 1.46-9.50 s over
twelve samples (median 3.09 s), all of it in stage2_apply_range().

Patch 1 adds kvm_stage2_unmap_all(), which skips spans covered by a zero
descriptor at least as large as an unmap chunk, keeps the tracked unmap
for everything else, removes the guest stage-2 records of skipped spans,
and ends with the VMID-wide TLBI that VMALLS12E1IS requires. It converts
the VMALLS12E1IS handler. Patch 2 converts kvm_nested_s2_unmap() (ALLE1IS,
whole-VM unmap) and the pending unmap of a recycled shadow MMU.

The series is based on master, which already has the guest stage-2
tracking. It overlaps with the unmerged kvm-arm64/unmap-vmall work, which
defines a kvm_stage2_unmap_all() with the same name and signature; if that
lands first, patch 1 becomes "skip empty spans in kvm_stage2_unmap_all()".
As far as I can tell, that WIP helper does not touch the guest stage-2
tracking records, which this series does.

All twelve Graviton4 samples, in ftrace order:

  22) $ 2278347 us  |  } /* handle_vmalls12e1is */
  21) $ 3229689 us  |  } /* handle_vmalls12e1is */
  13) $ 7244813 us  |  } /* handle_vmalls12e1is */
   1) $ 2950198 us  |  } /* handle_vmalls12e1is */
   6) $ 5785126 us  |  } /* handle_vmalls12e1is */
  25) $ 2484152 us  |  } /* handle_vmalls12e1is */
   4) $ 2505404 us  |  } /* handle_vmalls12e1is */
  16) $ 1464441 us  |  } /* handle_vmalls12e1is */
  28) $ 1935604 us  |  } /* handle_vmalls12e1is */
  19) $ 9495486 us  |  } /* handle_vmalls12e1is */
  14) $ 4151658 us  |  } /* handle_vmalls12e1is */
  18) $ 9223579 us  |  } /* handle_vmalls12e1is */

Tooling: the patches were drafted with AI assistance (SiemensGPT).

Testing: build-tested on the master branch and
boot-tested with Linux 6.18.29.