[PATCH 0/3] KVM: arm64: Stop stage-2 teardown scaling with the IPA size

[Fuad Tabba <fuad.tabba@linux.dev>]

Hi folks,

Two fixes for stage-2 teardown, which since 4ddfab5436b6 takes time
proportional to the size of the IPA space rather than to what is mapped.

Under QEMU TCG ('-cpu max', 16K pages with LPA2), destroying even an
empty VM takes about 7 seconds in pKVM. Without pKVM it takes about 19
seconds if the VM requests a 52-bit IPA (crosvm requests the host's
limit). I would expect it to scale the same way in hardware. I tested
only under QEMU, since I don't have hardware with LPA2.

pKVM gives every VM the host's IPA limit. The first makes it release the
VM's mappings from its tree instead of walking that space. My series
honoring KVM_VM_TYPE_ARM_IPA_SIZE in pKVM [1] (respinning it soon)
doesn't replace the fix, as crosvm requests the limit there too.

The second makes the non-pKVM teardown step over an empty entry larger
than the largest block in one go, rather than start a new walk for each
block-sized chunk of it. It depends on the first, as the lookup it adds
reads fields that hold the mapping tree on a pKVM stage-2.

The third moves the pKVM/non-pKVM split in the teardown into
kvm_vm_s2_ops, with no functional change. The first two apply back to
7.1, except that a backport of the second drops the flags argument 7.4
added to kvm_pgtable_get_leaf().

Based on Linux 7.4-rc1 (6628e7d4c6b25).

Cheers,
/fuad

[1] https://lore.kernel.org/all/20260917091826.2639205-1-fuad.tabba@linux.dev/

Fuad Tabba (3):
  KVM: arm64: Don't walk the whole IPA space when destroying a pKVM
    stage-2
  KVM: arm64: Skip empty IPA ranges when destroying stage-2 page-tables
  KVM: arm64: Move stage-2 teardown into kvm_vm_s2_ops

 arch/arm64/include/asm/kvm_host.h |  2 ++
 arch/arm64/kvm/mmu.c              | 34 +++++++++++++++++++++++--------
 arch/arm64/kvm/pkvm.c             | 17 ++++++++++++----
 3 files changed, 41 insertions(+), 12 deletions(-)


base-commit: 6628e7d4c6b25c8d18f1345e3a20d21829aae7e4
-- 
2.47.3