Re: [RFC PATCH 04/17] KVM: arm64: Protect page table traversal with RCU

[Ricardo Koller <ricarkol@google.com>]

On Fri, Apr 15, 2022 at 09:58:48PM +0000, Oliver Upton wrote:
> Use RCU to safely traverse the page tables in parallel; the tables
> themselves will only be freed from an RCU synchronized context. Don't
> even bother with adding support to hyp, and instead just assume
> exclusive access of the page tables.
> 
> Signed-off-by: Oliver Upton <oupton@google.com>
> ---
>  arch/arm64/kvm/hyp/pgtable.c | 23 ++++++++++++++++++++++-
>  1 file changed, 22 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> index 5b64fbca8a93..d4699f698d6e 100644
> --- a/arch/arm64/kvm/hyp/pgtable.c
> +++ b/arch/arm64/kvm/hyp/pgtable.c
> @@ -132,9 +132,28 @@ static kvm_pte_t kvm_phys_to_pte(u64 pa)
>  	return pte;
>  }
>  
> +
> +#if defined(__KVM_NVHE_HYPERVISOR__)
> +static inline void kvm_pgtable_walk_begin(void)
> +{}
> +
> +static inline void kvm_pgtable_walk_end(void)
> +{}
> +
> +#define kvm_dereference_ptep	rcu_dereference_raw
> +#else
> +#define kvm_pgtable_walk_begin	rcu_read_lock
> +
> +#define kvm_pgtable_walk_end	rcu_read_unlock
> +
> +#define kvm_dereference_ptep	rcu_dereference
> +#endif
> +
>  static kvm_pte_t *kvm_pte_follow(kvm_pte_t pte, struct kvm_pgtable_mm_ops *mm_ops)
>  {
> -	return mm_ops->phys_to_virt(kvm_pte_to_phys(pte));
> +	kvm_pte_t __rcu *ptep = mm_ops->phys_to_virt(kvm_pte_to_phys(pte));
> +
> +	return kvm_dereference_ptep(ptep);
>  }
>  
>  static void kvm_clear_pte(kvm_pte_t *ptep)
> @@ -288,7 +307,9 @@ int kvm_pgtable_walk(struct kvm_pgtable *pgt, u64 addr, u64 size,
>  		.walker	= walker,
>  	};
>  
> +	kvm_pgtable_walk_begin();
>  	return _kvm_pgtable_walk(&walk_data);
> +	kvm_pgtable_walk_end();

This might be fixed later in the series, but at this point the
rcu_read_unlock is never called.

>  }
>  
>  struct leaf_walk_data {
> -- 
> 2.36.0.rc0.470.gd361397f0d-goog
>