Re: [PATCH v4 12/12] KVM: Expose KVM_MEM_PRIVATE

[Maciej S. Szmigiero]

On 18.01.2022 14:21, Chao Peng wrote:
> KVM_MEM_PRIVATE is not exposed by default but architecture code can turn
> on it by implementing kvm_arch_private_memory_supported().
>
> Also private memslot cannot be movable and the same file+offset can not
> be mapped into different GFNs.
>
> Signed-off-by: Yu Zhang <yu.c.zhang@xxxxxxxxxxxxxxx>
> Signed-off-by: Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx>
> ---
(..)
>   
>   static bool kvm_check_memslot_overlap(struct kvm_memslots *slots, int id,
> -				      gfn_t start, gfn_t end)
> +				      struct file *file,
> +				      gfn_t start, gfn_t end,
> +				      loff_t start_off, loff_t end_off)
>   {
>   	struct kvm_memslot_iter iter;
> +	struct kvm_memory_slot *slot;
> +	struct inode *inode;
> +	int bkt;
>   
>   	kvm_for_each_memslot_in_gfn_range(&iter, slots, start, end) {
>   		if (iter.slot->id != id)
>   			return true;
>   	}
>   
> +	/* Disallow mapping the same file+offset into multiple gfns. */
> +	if (file) {
> +		inode = file_inode(file);
> +		kvm_for_each_memslot(slot, bkt, slots) {
> +			if (slot->private_file &&
> +			     file_inode(slot->private_file) == inode &&
> +			     !(end_off <= slot->private_offset ||
> +			       start_off >= slot->private_offset
> +					     + (slot->npages >> PAGE_SHIFT)))
> +				return true;
> +		}
> +	}

That's a linear scan of all memslots on each CREATE (and MOVE) operation
with a fd - we just spent more than a year rewriting similar linear scans
into more efficient operations in KVM.

Thanks,
Maciej