Re: [PATCH v6 02/11] KVM: x86: Relax locking for kvm_test_age_gfn and kvm_age_gfn

On Thu, Jul 25, 2024 at 11:08 AM David Matlack [off-list ref] wrote:

On 2024-07-24 01:10 AM, James Houghton wrote:

quoted

Walk the TDP MMU in an RCU read-side critical section. This requires a
way to do RCU-safe walking of the tdp_mmu_roots; do this with a new
macro. The PTE modifications are now done atomically, and
kvm_tdp_mmu_spte_need_atomic_write() has been updated to account for the
fact that kvm_age_gfn can now lockless update the accessed bit and the
R/X bits).

If the cmpxchg for marking the spte for access tracking fails, we simply
retry if the spte is still a leaf PTE. If it isn't, we return false
to continue the walk.

Harvesting age information from the shadow MMU is still done while
holding the MMU write lock.

Suggested-by: Yu Zhao <redacted>
Signed-off-by: James Houghton <redacted>

Aside from the comment fixes below,

Reviewed-by: David Matlack <dmatlack@google.com>

Thank you!

quoted

---
 arch/x86/include/asm/kvm_host.h |  1 +
 arch/x86/kvm/Kconfig            |  1 +
 arch/x86/kvm/mmu/mmu.c          | 10 ++++-
 arch/x86/kvm/mmu/tdp_iter.h     | 27 +++++++------
 arch/x86/kvm/mmu/tdp_mmu.c      | 67 +++++++++++++++++++++++++--------
 5 files changed, 77 insertions(+), 29 deletions(-)

[...]

quoted

--- a/arch/x86/kvm/mmu/tdp_iter.h
+++ b/arch/x86/kvm/mmu/tdp_iter.h

@@ -25,6 +25,13 @@ static inline u64 kvm_tdp_mmu_write_spte_atomic(tdp_ptep_t sptep, u64 new_spte)
      return xchg(rcu_dereference(sptep), new_spte);
 }

+static inline u64 tdp_mmu_clear_spte_bits_atomic(tdp_ptep_t sptep, u64 mask)
+{
+     atomic64_t *sptep_atomic = (atomic64_t *)rcu_dereference(sptep);
+
+     return (u64)atomic64_fetch_and(~mask, sptep_atomic);
+}
+
 static inline void __kvm_tdp_mmu_write_spte(tdp_ptep_t sptep, u64 new_spte)
 {
      KVM_MMU_WARN_ON(is_ept_ve_possible(new_spte));

@@ -32,10 +39,11 @@ static inline void __kvm_tdp_mmu_write_spte(tdp_ptep_t sptep, u64 new_spte)
 }

 /*
- * SPTEs must be modified atomically if they are shadow-present, leaf
- * SPTEs, and have volatile bits, i.e. has bits that can be set outside
- * of mmu_lock.  The Writable bit can be set by KVM's fast page fault
- * handler, and Accessed and Dirty bits can be set by the CPU.
+ * SPTEs must be modified atomically if they have bits that can be set outside
+ * of the mmu_lock. This can happen for any shadow-present leaf SPTEs, as the
+ * Writable bit can be set by KVM's fast page fault handler, the Accessed and
+ * Dirty bits can be set by the CPU, and the Accessed and R/X bits can be

"R/X bits" should be "W/R/X bits".

Thanks. Right, we are clearing all of VMX_EPT_RWX_MASK.

quoted

+ * cleared by age_gfn_range.

nit: "age_gfn_range()"

Thanks, will fix this and all the other places where I've left off the ().