Re: [PATCH v12 06/46] arm64: RMI: Define the user ABI

[Marc Zyngier <maz@kernel.org>]

On Mon, 02 Mar 2026 15:23:44 +0000,
Steven Price <steven.price@arm.com> wrote:
> 
> >> +  struct kvm_arm_rmi_populate {
> >> +	__u64 base;
> >> +	__u64 size;
> >> +	__u64 source_uaddr;
> >> +	__u32 flags;
> >> +	__u32 reserved;
> >> +  };
> >> +
> >> +Populate a region of protected address space by copying the data from the user
> >> +space pointer provided. This is only valid before any VCPUs have been run.
> >> +The ioctl might not populate the entire region and user space may have to
> >> +repeatedly call it (with updated pointers) to populate the entire region.
> > 
> > size as a __u64 is odd, as the return value from the ioctl is a signed
> > int. This implies that you can't really report how many bytes you have
> > copied.  Some form of consistency wouldn't hurt.
> 
> Good spot. In practice this works because >2GB in one operation is
> highly unlikely to be processed in one go. But I guess I'll change this
> to have an output size argument. I guess I could make the kernel update
> all of base,size,source_uaddr which would simplify user space.

In a conversation with Suzuki, I suggested that splice(2) could be a
nicer way to express this, and allow asynchronous use with io-uring.

After all, having a guestmem backend for CCA is not exactly
outlandish, and having a splice implementation realistic enough.

Thoughts?

	M.

-- 
Jazz isn't dead. It just smells funny.