[PATCH] KVM: arm64: Don't split hugepages outside of MMU write lock

[Oliver Upton]

It is possible to take a stage-2 permission fault on a page larger than
PAGE_SIZE. For example, when running a guest backed by 2M HugeTLB, KVM
eagerly maps at the largest possible block size. When dirty logging is
enabled on a memslot, KVM does *not* eagerly split these 2M stage-2
mappings and instead clears the write bit on the pte.

Since dirty logging is always performed at PAGE_SIZE granularity, KVM
lazily splits these 2M block mappings down to PAGE_SIZE in the stage-2
fault handler. This operation must be done under the write lock. Since
commit f783ef1c0e82 ("KVM: arm64: Add fast path to handle permission
relaxation during dirty logging"), the stage-2 fault handler
conditionally takes the read lock on permission faults with dirty
logging enabled. To that end, it is possible to split a 2M block mapping
while only holding the read lock.

The problem is demonstrated by running kvm_page_table_test with 2M
anonymous HugeTLB, which splats like so:

  WARNING: CPU: 5 PID: 15276 at arch/arm64/kvm/hyp/pgtable.c:153 stage2_map_walk_leaf+0x124/0x158

  [...]

  Call trace:
  stage2_map_walk_leaf+0x124/0x158
  stage2_map_walker+0x5c/0xf0
  __kvm_pgtable_walk+0x100/0x1d4
  __kvm_pgtable_walk+0x140/0x1d4
  __kvm_pgtable_walk+0x140/0x1d4
  kvm_pgtable_walk+0xa0/0xf8
  kvm_pgtable_stage2_map+0x15c/0x198
  user_mem_abort+0x56c/0x838
  kvm_handle_guest_abort+0x1fc/0x2a4
  handle_exit+0xa4/0x120
  kvm_arch_vcpu_ioctl_run+0x200/0x448
  kvm_vcpu_ioctl+0x588/0x664
  __arm64_sys_ioctl+0x9c/0xd4
  invoke_syscall+0x4c/0x144
  el0_svc_common+0xc4/0x190
  do_el0_svc+0x30/0x8c
  el0_svc+0x28/0xcc
  el0t_64_sync_handler+0x84/0xe4
  el0t_64_sync+0x1a4/0x1a8

Fix the issue by only acquiring the read lock if the guest faulted on a
PAGE_SIZE granule w/ dirty logging enabled. Since it is possible for the
faulting IPA to get collapsed into a larger block mapping until the read
lock is acquired, retry the faulting instruction any time that the fault
cannot be fixed by relaxing permissions. In so doing, the fault handler
will acquire the write lock for the subsequent fault on a larger
PAGE_SIZE mapping and split the block safely behind the write lock.

Fixes: f783ef1c0e82 ("KVM: arm64: Add fast path to handle permission relaxation during dirty logging")
Cc: Jing Zhang <jingzhangos at google.com>
Signed-off-by: Oliver Upton <oupton at google.com>
---

Applies cleanly to kvmarm/fixes at the following commit:

  8872d9b3e35a ("KVM: arm64: Drop unneeded minor version check from PSCI v1.x handler")

Tested the patch by running KVM selftests. Additionally, I did 10
iterations of the kvm_page_table_test with 2M anon HugeTLB memory.

It is expected that this patch will cause fault serialization in the
pathological case where all vCPUs are faulting on the same granule of
memory, as every vCPU will attempt to acquire the write lock. The only
safe way to cure this contention is to dissolve pages eagerly outside of
the stage-2 fault handler (like x86).

 arch/arm64/kvm/mmu.c | 25 ++++++++++++++++++++-----
 1 file changed, 20 insertions(+), 5 deletions(-)

diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index 0d19259454d8..9384325bf3df 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -1079,7 +1079,7 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
 	gfn_t gfn;
 	kvm_pfn_t pfn;
 	bool logging_active = memslot_is_logging(memslot);
-	bool logging_perm_fault = false;
+	bool use_read_lock = false;
 	unsigned long fault_level = kvm_vcpu_trap_get_fault_level(vcpu);
 	unsigned long vma_pagesize, fault_granule;
 	enum kvm_pgtable_prot prot = KVM_PGTABLE_PROT_R;
@@ -1114,7 +1114,8 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
 	if (logging_active) {
 		force_pte = true;
 		vma_shift = PAGE_SHIFT;
-		logging_perm_fault = (fault_status == FSC_PERM && write_fault);
+		use_read_lock = (fault_status == FSC_PERM && write_fault &&
+				 fault_granule == PAGE_SIZE);
 	} else {
 		vma_shift = get_vma_page_shift(vma, hva);
 	}
@@ -1218,7 +1219,7 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
 	 * logging dirty logging, only acquire read lock for permission
 	 * relaxation.
 	 */
-	if (logging_perm_fault)
+	if (use_read_lock)
 		read_lock(&kvm->mmu_lock);
 	else
 		write_lock(&kvm->mmu_lock);
@@ -1267,10 +1268,24 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
 	 */
 	if (fault_status == FSC_PERM && vma_pagesize == fault_granule) {
 		ret = kvm_pgtable_stage2_relax_perms(pgt, fault_ipa, prot);
-	} else {
+	} else if (!use_read_lock) {
 		ret = kvm_pgtable_stage2_map(pgt, fault_ipa, vma_pagesize,
 					     __pfn_to_phys(pfn), prot,
 					     memcache);
+
+	/*
+	 * The read lock is taken if the FSC indicates that the guest faulted on
+	 * a PAGE_SIZE granule. It is possible that the stage-2 fault raced with
+	 * a map operation that collapsed the faulted address into a larger
+	 * block mapping.
+	 *
+	 * Since KVM splits mappings down to PAGE_SIZE when dirty logging is
+	 * enabled, it is necessary to hold the write lock for faults where
+	 * fault_granule > PAGE_SIZE. Retry the faulting instruction and acquire
+	 * the write lock on the next exit.
+	 */
+	} else {
+		ret = -EAGAIN;
 	}
 
 	/* Mark the page dirty only if the fault is handled successfully */
@@ -1280,7 +1295,7 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
 	}
 
 out_unlock:
-	if (logging_perm_fault)
+	if (use_read_lock)
 		read_unlock(&kvm->mmu_lock);
 	else
 		write_unlock(&kvm->mmu_lock);
-- 
2.35.1.1094.g7c7d902a7c-goog