Re: [Xen-devel] [PATCH 2/3] amd-iommu: disable iommu_hap_pt_share with AMD IOMMUs

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 07.04.14 at 18:02, <roger.pau@xxxxxxxxxx> wrote:
> --- a/xen/drivers/passthrough/amd/iommu_init.c
> +++ b/xen/drivers/passthrough/amd/iommu_init.c
> @@ -1255,6 +1255,13 @@ int __init amd_iommu_init(void)
>      if ( iterate_ivrs_mappings(amd_iommu_setup_device_table) != 0 )
>          goto error_out;
>  
> +    /*
> +     * Disable sharing HAP page tables with AMD IOMMU,
> +     * since it only supports p2m_ram_rw, and this would
> +     * prevent doing IO to/from mapped grant frames.
> +     */
> +    iommu_hap_pt_share = 0;
> +

I guess at the very least when the option was specified on the
command line you should issue a warning, or perhaps even stay
away from enforcing this.

I also think we ought to consider alternatives before taking a harsh
step like this (and that's independent of my earlier indication that we
may want to switch away from sharing these tables): It looks like the
global bit in the host page tables is unused and ignored, and could
therefore be used to indicate grant entries. The IW bit could be
used to distinguish p2m_{ram,grant}_{ro,rw} respectively.

Curious what Tim's thoughts here are...

Jan


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