Re: [PATCH v2 08/15] KVM: arm64: Protect stage-2 traversal with RCU

[Sean Christopherson <seanjc@google.com>]

On Fri, Oct 07, 2022, Oliver Upton wrote:
> The use of RCU is necessary to safely change the stage-2 page tables in
> parallel. Acquire and release the RCU read lock when traversing the page
> tables.
> 
> Use the _raw() flavor of rcu_dereference when changes to the page tables
> are otherwise protected from parallel software walkers (e.g. holding the
> write lock).
> 
> Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
> ---

...

> @@ -32,6 +39,33 @@ static inline kvm_pte_t *kvm_dereference_pteref(kvm_pteref_t pteref, bool shared
>  	return pteref;
>  }
>  
> +static inline void kvm_pgtable_walk_begin(void) {}
> +static inline void kvm_pgtable_walk_end(void) {}
> +
> +#else
> +
> +typedef kvm_pte_t __rcu *kvm_pteref_t;
> +
> +static inline kvm_pte_t *kvm_dereference_pteref(kvm_pteref_t pteref, bool shared)
> +{
> +	if (shared)
> +		return rcu_dereference(pteref);
> +
> +	return rcu_dereference_raw(pteref);

Rather than use raw, use rcu_dereference_check().  If you can plumb down @kvm or
@mmu_lock, the ideal check would be (apparently there's no lockdep_is_held_write()
wrapper?)

	return READ_ONCE(*rcu_dereference_check(ptep, lockdep_is_held_type(mmu_lock, 0)));

If getting at mmu_lock is too difficult, this can still be

	return READ_ONCE(*rcu_dereference_check(ptep, !shared);

Doubt it matters for code generation, but IMO it's cleaner overall.