[PATCH v3 10/28] KVM: x86/mmu: Add helpers to read/write TDP MMU SPTEs and document RCU

Sean Christopherson
2022-02-26T00:17:10Z

Add helpers to read and write TDP MMU SPTEs instead of open coding
rcu_dereference() all over the place, and to provide a convenient
location to document why KVM doesn't exempt holding mmu_lock for write
from having to hold RCU (and any future changes to the rules).

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Ben Gardon <bgardon@google.com>
---
arch/x86/kvm/mmu/tdp_iter.c | 6 +++---
arch/x86/kvm/mmu/tdp_iter.h | 16 ++++++++++++++++
arch/x86/kvm/mmu/tdp_mmu.c | 6 +++---
3 files changed, 22 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kvm/mmu/tdp_iter.c b/arch/x86/kvm/mmu/tdp_iter.c
index be3f096db2eb..6d3b3e5a5533 100644
--- a/arch/x86/kvm/mmu/tdp_iter.c
+++ b/arch/x86/kvm/mmu/tdp_iter.c
@@ -12,7 +12,7 @@ static void tdp_iter_refresh_sptep(struct tdp_iter *iter)
{
iter->sptep = iter->pt_path[iter->level - 1] +
SHADOW_PT_INDEX(iter->gfn << PAGE_SHIFT, iter->level);
- iter->old_spte = READ_ONCE(*rcu_dereference(iter->sptep));
+ iter->old_spte = kvm_tdp_mmu_read_spte(iter->sptep);
}

static gfn_t round_gfn_for_level(gfn_t gfn, int level)
@@ -89,7 +89,7 @@ static bool try_step_down(struct tdp_iter *iter)
* Reread the SPTE before stepping down to avoid traversing into page
* tables that are no longer linked from this entry.
*/
- iter->old_spte = READ_ONCE(*rcu_dereference(iter->sptep));
+ iter->old_spte = kvm_tdp_mmu_read_spte(iter->sptep);

child_pt = spte_to_child_pt(iter->old_spte, iter->level);
if (!child_pt)
@@ -123,7 +123,7 @@ static bool try_step_side(struct tdp_iter *iter)
iter->gfn += KVM_PAGES_PER_HPAGE(iter->level);
iter->next_last_level_gfn = iter->gfn;
iter->sptep++;
- iter->old_spte = READ_ONCE(*rcu_dereference(iter->sptep));
+ iter->old_spte = kvm_tdp_mmu_read_spte(iter->sptep);

return true;
}
diff --git a/arch/x86/kvm/mmu/tdp_iter.h b/arch/x86/kvm/mmu/tdp_iter.h
index 216ebbe76ddd..bb9b581f1ee4 100644
--- a/arch/x86/kvm/mmu/tdp_iter.h
+++ b/arch/x86/kvm/mmu/tdp_iter.h
@@ -9,6 +9,22 @@

typedef u64 __rcu *tdp_ptep_t;

+/*
+ * TDP MMU SPTEs are RCU protected to allow paging structures (non-leaf SPTEs)
+ * to be zapped while holding mmu_lock for read. Holding RCU isn't required for
+ * correctness if mmu_lock is held for write, but plumbing "struct kvm" down to
+ * the lower depths of the TDP MMU just to make lockdep happy is a nightmare, so
+ * all accesses to SPTEs are done under RCU protection.
+ */
+static inline u64 kvm_tdp_mmu_read_spte(tdp_ptep_t sptep)
+{
+ return READ_ONCE(*rcu_dereference(sptep));
+}
+static inline void kvm_tdp_mmu_write_spte(tdp_ptep_t sptep, u64 val)
+{
+ WRITE_ONCE(*rcu_dereference(sptep), val);
+}
+
/*
* A TDP iterator performs a pre-order walk over a TDP paging structure.
*/
diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index 4f460782a848..8fbf3364f116 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -609,7 +609,7 @@ static inline int tdp_mmu_zap_spte_atomic(struct kvm *kvm,
* here since the SPTE is going from non-present
* to non-present.
*/
- WRITE_ONCE(*rcu_dereference(iter->sptep), 0);
+ kvm_tdp_mmu_write_spte(iter->sptep, 0);

return 0;
}
@@ -648,7 +648,7 @@ static inline void __tdp_mmu_set_spte(struct kvm *kvm, struct tdp_iter *iter,
*/
WARN_ON(is_removed_spte(iter->old_spte));

- WRITE_ONCE(*rcu_dereference(iter->sptep), new_spte);
+ kvm_tdp_mmu_write_spte(iter->sptep, new_spte);

__handle_changed_spte(kvm, iter->as_id, iter->gfn, iter->old_spte,
new_spte, iter->level, false);
@@ -1046,7 +1046,7 @@ int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
* because the new value informs the !present
* path below.
*/
- iter.old_spte = READ_ONCE(*rcu_dereference(iter.sptep));
+ iter.old_spte = kvm_tdp_mmu_read_spte(iter.sptep);
}

if (!is_shadow_present_pte(iter.old_spte)) {
--
2.35.1.574.g5d30c73bfb-goog