Re: [PATCH] xen-blkback: only attach blkback if the required features are met

[David Vrabel <david.vrabel@citrix.com>]

On 04/04/14 15:41, Roger Pau Monne wrote:
> Blkback cannot work properly on auto-translated guests if Xen doesn't
> update the IOMMU when performing grant maps/unmaps, so only attach if
> the newly introduced XENFEAT_hvm_gntmap_supports_iommu is found.

Can you explain the problem in more detail and which guest
configurations are affected?

This isn't a problem that is specific to blkback, but any backend that
grant maps and passes the foreign pages to a device so there needs to be
a generic solution.

Why can't this be fixed by having the swiotlb bounce foreign pages?

> --- a/drivers/block/xen-blkback/blkback.c
> +++ b/drivers/block/xen-blkback/blkback.c
> @@ -1381,6 +1381,18 @@ static int __init xen_blkif_init(void)
>  	if (!xen_domain())
>  		return -ENODEV;
>  
> +#ifdef CONFIG_X86
> +	if (xen_feature(XENFEAT_auto_translated_physmap) &&
> +	    !xen_feature(XENFEAT_hvm_gntmap_supports_iommu)) {
> +		/*
> +		 * blkback cannot work properly on auto-translated guests
> +		 * if grant table mappings don't update the IOMMU entries.
> +		 */
> +		pr_debug(DRV_PFX "Disabling blkback because Xen is missing feature XENFEAT_hvm_gntmap_supports_iommu\n");
> +		return -ENODEV;
> +	}
> +#endif /* CONFIG_X86 */

Why CONFIG_X86?

David