[PATCH 2/3] KVM: arm64: Skip empty IPA ranges when destroying stage-2 page-tables

[Fuad Tabba]

The page-table walker steps over an empty entry in one go, but
kvm_stage2_destroy() starts a new walk for every block-sized chunk of
the IPA space. Tearing down a VM that requests a 52-bit IPA then takes
2^27 walks with 16K pages (2^22 with 4K), however little it maps.
crosvm requests the host's limit.

After destroying each chunk, look up the entry at its start and, if it
is empty and larger than a chunk, skip its whole range. The lookup reads
fields that hold the mapping tree on a pKVM stage-2, so this depends on
the previous patch keeping pKVM out of stage2_destroy_range(). For the
same reason, call kvm_pgtable_stage2_destroy_range() directly rather
than through KVM_PGT_FN().

Fixes: 4ddfab5436b69 ("KVM: arm64: Reschedule as needed when destroying the stage-2 page-tables")
Signed-off-by: Fuad Tabba <fuad.tabba at linux.dev>
---
 arch/arm64/kvm/mmu.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index 42977cf8a7f5d..10b425a0141be 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -1010,9 +1010,20 @@ static void stage2_destroy_range(struct kvm_pgtable *pgt, phys_addr_t addr,
 	u64 next;
 
 	do {
+		kvm_pte_t pte;
+		s8 level;
+
 		next = stage2_range_addr_end(addr, end);
-		KVM_PGT_FN(kvm_pgtable_stage2_destroy_range)(pgt, addr,
-							     next - addr);
+		kvm_pgtable_stage2_destroy_range(pgt, addr, next - addr);
+
+		/*
+		 * The walk freed every table it emptied, so skipping an empty
+		 * entry here doesn't leave anything behind.
+		 */
+		if (!kvm_pgtable_get_leaf(pgt, addr, &pte, &level, 0) && !pte &&
+		    level < KVM_PGTABLE_MIN_BLOCK_LEVEL)
+			next = __stage2_range_addr_end(addr, end, kvm_granule_size(level));
+
 		if (next != end)
 			cond_resched();
 	} while (addr = next, addr != end);
-- 
2.47.3