Re: [PATCH v3 12/15] util: vfio-helpers: Implement ram_block_resized()

[Peter Xu]

On Thu, Feb 27, 2020 at 11:12:02AM +0100, David Hildenbrand wrote:
> Let's implement ram_block_resized(), allowing resizeable mappings.
> 
> For resizeable mappings, we reserve $max_size IOVA address space, but only
> map $size of it. When resizing, unmap the old part and remap the new
> part. We'll need a new ioctl to do this atomically (e.g., to resize
> while the guest is running - not allowed for now).

Curious: I think it's true for now because resizing only happens
during reboot or destination VM during migration (but before
switching).  However is that guaranteed too in the future?

[...]

> @@ -631,7 +658,7 @@ int qemu_vfio_dma_map(QEMUVFIOState *s, void *host, 
> size_t size,
>                  qemu_vfio_remove_mapping(s, mapping);
>                  goto out;
>              }
> -            s->low_water_mark += size;
> +            s->low_water_mark += max_size;

I think it's fine to only increase the low water mark here, however
imo it would be better to also cache the max size in IOVAMapping too,
then in resize() we double check new_size <= max_size?  It also makes
IOVAMapping more self contained.

Thanks,

-- 
Peter Xu