Re: [PATCH v2 01/15] KVM: Reinstall old memslots if arch preparation fails

[Christoffer Dall <christoffer.dall@arm.com>]

On Mon, Oct 21, 2019 at 05:35:23PM -0700, Sean Christopherson wrote:
> Reinstall the old memslots if preparing the new memory region fails
> after invalidating a to-be-{re}moved memslot.
> 
> Remove the superfluous 'old_memslots' variable so that it's somewhat
> clear that the error handling path needs to free the unused memslots,
> not simply the 'old' memslots.
> 
> Fixes: bc6678a33d9b9 ("KVM: introduce kvm->srcu and convert kvm_set_memory_region to SRCU update")
> Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
> ---
>  virt/kvm/kvm_main.c | 23 ++++++++++++-----------
>  1 file changed, 12 insertions(+), 11 deletions(-)
> 
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 67ef3f2e19e8..9afd706dc038 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -924,7 +924,7 @@ int __kvm_set_memory_region(struct kvm *kvm,
>  	unsigned long npages;
>  	struct kvm_memory_slot *slot;
>  	struct kvm_memory_slot old, new;
> -	struct kvm_memslots *slots = NULL, *old_memslots;
> +	struct kvm_memslots *slots;
>  	int as_id, id;
>  	enum kvm_mr_change change;
>  
> @@ -1032,7 +1032,13 @@ int __kvm_set_memory_region(struct kvm *kvm,
>  		slot = id_to_memslot(slots, id);
>  		slot->flags |= KVM_MEMSLOT_INVALID;
>  
> -		old_memslots = install_new_memslots(kvm, as_id, slots);
> +		/*
> +		 * We can re-use the old memslots, the only difference from the
> +		 * newly installed memslots is the invalid flag, which will get
> +		 * dropped by update_memslots anyway.  We'll also revert to the
> +		 * old memslots if preparing the new memory region fails.
> +		 */
> +		slots = install_new_memslots(kvm, as_id, slots);
>  
>  		/* From this point no new shadow pages pointing to a deleted,
>  		 * or moved, memslot will be created.
> @@ -1042,13 +1048,6 @@ int __kvm_set_memory_region(struct kvm *kvm,
>  		 *	- kvm_is_visible_gfn (mmu_check_roots)
>  		 */
>  		kvm_arch_flush_shadow_memslot(kvm, slot);
> -
> -		/*
> -		 * We can re-use the old_memslots from above, the only difference
> -		 * from the currently installed memslots is the invalid flag.  This
> -		 * will get overwritten by update_memslots anyway.
> -		 */
> -		slots = old_memslots;
>  	}
>  
>  	r = kvm_arch_prepare_memory_region(kvm, &new, mem, change);
> @@ -1062,15 +1061,17 @@ int __kvm_set_memory_region(struct kvm *kvm,
>  	}
>  
>  	update_memslots(slots, &new, change);
> -	old_memslots = install_new_memslots(kvm, as_id, slots);
> +	slots = install_new_memslots(kvm, as_id, slots);
>  
>  	kvm_arch_commit_memory_region(kvm, mem, &old, &new, change);
>  
>  	kvm_free_memslot(kvm, &old, &new);
> -	kvfree(old_memslots);
> +	kvfree(slots);
>  	return 0;
>  
>  out_slots:
> +	if (change == KVM_MR_DELETE || change == KVM_MR_MOVE)
> +		slots = install_new_memslots(kvm, as_id, slots);
>  	kvfree(slots);
>  out_free:
>  	kvm_free_memslot(kvm, &new, &old);
> -- 
> 2.22.0

Reviewed-by: Christoffer Dall <christoffer.dall@arm.com>