[PATCH v5 3/7] KVM: arm64: Top up stage-2 memcache for dirty logging faults

[Fuad Tabba]

From: Bradley Morgan <include at grrlz.net>

Dirty logging forces new stage-2 mappings to page size but does not
always split an existing block first (eager splitting is best effort
and off by default). A non-write permission fault on such a block, an
instruction fetch, still needs a page-table page to split it, but the
top-up is gated on write faults.

With the cache empty, kvm_mmu_memory_cache_alloc() hits its
guest-triggerable WARN_ON(!nobjs) and falls back to a GFP_ATOMIC
allocation under mmu_lock, with a BUG_ON() if that fails.

Top up the memcache for any permission fault while dirty logging is
active.

Fixes: 6f745f1bb5bf ("KVM: arm64: Convert user_mem_abort() to generic page-table API")
Link: https://lore.kernel.org/all/20260623165634.699011F000E9@smtp.kernel.org/
Signed-off-by: Bradley Morgan <include at grrlz.net>
[tabba: reword the commit message for the generic, non-pKVM failure mode]
Signed-off-by: Fuad Tabba <fuad.tabba at linux.dev>
---
 arch/arm64/kvm/mmu.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index 4d7c9bdcef00..74e7e7f7564c 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -2113,14 +2113,13 @@ static int user_mem_abort(const struct kvm_s2_fault_desc *s2fd)
 	 * Permission faults just need to update the existing leaf entry,
 	 * and so normally don't require allocations from the memcache. The
 	 * only exception to this is when dirty logging is enabled at runtime
-	 * and a write fault needs to collapse a block entry into a table.
+	 * and a fault needs to collapse a block entry into a table.
 	 * Under pKVM a permission fault can also collapse pages into a block,
 	 * which needs a fresh mapping object, and the hypervisor requires the
 	 * min-pages memcache even when the install allocates nothing.
 	 */
 	memcache = get_mmu_memcache(s2fd->vcpu);
-	if (!perm_fault || (memslot_is_logging(s2fd->memslot) &&
-			    kvm_is_write_fault(s2fd->vcpu)) ||
+	if (!perm_fault || memslot_is_logging(s2fd->memslot) ||
 	    is_protected_kvm_enabled()) {
 		ret = topup_mmu_memcache(s2fd->vcpu, memcache);
 		if (ret)
-- 
2.39.5