Re: [PATCH RFC] p2m: p2m_mmio_direct set RW permissions

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 22/01/15 09:53, Jan Beulich wrote:
>>>> On 21.01.15 at 21:55, <elena.ufimtseva@oracle.com> wrote:
>> p2m_mmio_direct should result in setting IOMMUF_readable and IOMMUF_writable
>> flags.
>> When pvh domain maps mmio regions, the EPT entries are not getting mapped.
>> This leads to IOMMU Page faults for some devices, as for example USB Host
>> controllers with embedded Debug devices. See pvh-set-need_iommu-early RFC
>> patch discussion fgor detail.
> Even more so that the two patches aren't even a series, that part
> of the description belongs here, not in the other patch.
>
>> I will appreciate your comments and ideas in regards to this change.
>>
>> Looking at Roger patches (xen/pvh: check permissions when adding MMIO 
>> regions)
>> the mmio memory type is proposed to be changed from p2m_mmio_direct to 
>> p2m_access_rw.
>> This type still does not have proper IOMMU flags mapping.
> A fundamental question is what business devices have to DMA their
> own (or other devices') MMIO space. I could remotely see a need
> for this for e.g. frame buffers, but I have difficulty understanding
> this for USB devices. Please at the very least provide details on the
> MMIO regions that those devices have, and which of them you
> observed IOMMU faults on.

It would appear that, in this case, it is a USB controller lacking an
appropriate RMRR description in the ACPI tables.

I feel it would be better to have a command line option to add extra
RMRRs, similar to ivrs_{hpet,ioapic} for AMD firmware issues.  Of
course, this would also rely on the RMRR series currently still in design.

~Andrew