Re: [PATCH 1/3] KVM: arm64: vgic-its: Don't dereference a NULL collection on ITT save

Hi Fuad,

Thanks for fixing this.

On Wed, Aug 05, 2026 at 10:38:26AM +0100, Fuad Tabba wrote:
> @@ -2119,6 +2119,14 @@ static int vgic_its_save_ite(struct vgic_its *its, struct its_device *dev,
>  	u32 next_offset;
>  	u64 val;
>  
> +	/*
> +	 * MAPC with V=0 keeps the ITEs mapped but drops their collection,
> +	 * and with it the ICID. Save a zeroed entry, which the restore path
> +	 * reads back as invalid.
> +	 */
> +	if (!ite->collection)
> +		return vgic_its_write_entry_lock(its, gpa, 0ULL, ite);
> +

Writing a zero entry results in a suboptimal scan on the target,
compute_next_eventid_offset() should just skip ITEs that are known
invalid. Or better yet, we should just make the ITS emulation consistent
with the migration behavior and delete ITEs when their corresponding
collection is deleted. As you note, this is UNPRED after all :)

Thanks,
Oliver