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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

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

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.

Roger.


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