Re: [Xen-devel] [PATCH RFC] p2m: p2m_mmio_direct set RW permissions

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 22.01.15 at 12:37, <roger.pau@xxxxxxxxxx> wrote:
> El 22/01/15 a les 12.09, Jan Beulich ha escrit:
>>>>> On 22.01.15 at 11:59, <andrew.cooper3@xxxxxxxxxx> wrote:
>>> On 22/01/15 09:53, Jan Beulich wrote:
>>>>>>> On 21.01.15 at 21:55, <elena.ufimtseva@xxxxxxxxxx> 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.
>> 
>> No, RMRRs only reference RAM pages afaik.
> 
> According to Linux IOMMU support document RMRRs reference regions marked
> as "reserved" in the e820 memory map:
> 
> https://www.kernel.org/doc/Documentation/Intel-IOMMU.txt 

Exactly. And MMIO PCI BARs in particular are never to be
reflected in the E820 (not the least because they're relocatable).

> I don't think we are setting proper IOMMU entries for this regions at
> all with the current PVH Dom0 code. IMHO instead of just adding IOMMU
> entries for every MMIO region we should just add IOMMU entries for the
> RMRR regions.

Yes, RMRR regions should certainly be put there as r/w entries.

Jan


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