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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 08.04.14 at 16:16, <tim@xxxxxxx> wrote:
> At 09:52 +0100 on 08 Apr (1396947164), Jan Beulich wrote:
>> >>> 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...
> 
> Happy to use the global bit to mark grant entries (though I haven't
> checked the docs to be sure it's actually safe). OTOH it seems
> like the hadres IOMMU tables might be going away for other reasons so
> I'm not sure it's worth putting a lot of effort into being clever
> around this.

Yeah - I simply wanted to make sure we think of alternatives if
they make sense; if they don't, and if we're settled to drop
shared tables anyway, then the patch is presumably fine as is.

Jan


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