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

[David Matlack <dmatlack@google.com>]

On 2024-07-24 01:10 AM, James Houghton wrote:
> 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 <yuzhao@google.com>
> Signed-off-by: James Houghton <jthoughton@google.com>

Aside from the comment fixes below,

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

> ---
>  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(-)
> 
[...]
> --- 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".

> + * cleared by age_gfn_range.

nit: "age_gfn_range()"