Re: [Xen-devel] [PATCH v6 01/14] iommu: introduce the concept of BFN...

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 04.09.18 at 10:49, <Paul.Durrant@xxxxxxxxxx> wrote:
>>  -----Original Message-----
>> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
>> Sent: 04 September 2018 09:47
>> To: Kevin Tian <kevin.tian@xxxxxxxxx>
>> Cc: Suravee Suthikulpanit <suravee.suthikulpanit@xxxxxxx>; Julien Grall
>> <julien.grall@xxxxxxx>; Paul Durrant <Paul.Durrant@xxxxxxxxxx>; Stefano
>> Stabellini <sstabellini@xxxxxxxxxx>; xen-devel <xen-
>> devel@xxxxxxxxxxxxxxxxxxxx>
>> Subject: Re: [Xen-devel] [PATCH v6 01/14] iommu: introduce the concept of
>> BFN...
>> 
>> >>> On 04.09.18 at 10:37, <kevin.tian@xxxxxxxxx> wrote:
>> >>  From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
>> >> Sent: Tuesday, September 4, 2018 4:33 PM
>> >> >
>> >> > bus address is commonly used along with physical/virtual address, to
>> >> > represent different views between devices and CPU. From that angle
>> >> > I think BFN is a clear term in this context. btw it is not necessary to
>> >> > differentiate GBFN and MBFN since there is only one BFN view per
>> >> > device.
>> >>
>> >> Sure, but you neglect the presence of one or more IOMMUs when
>> >> you say "between devices and CPU". There addresses prior to and
>> >> after IOMMU translation are distinct, and while the one before the
>> >> translation matches the device view, the one after translation does
>> >> not necessarily match the CPU view. Hence there are two "bus"
>> >> frame numbers here - one representing the device view, and the
>> >> other representing the IOMMU (output) view.
>> >>
>> >
>> > I didn't get. the output address from IOMMU is the one sent to
>> > memory controller, same as the one sent from CPU.
>> 
>> That's on present x86 systems, but aiui not in the general case. The
>> terminology to be used in Xen should fit the general case though.
> 
> So your concern is cascaded IOMMUs?

Not primarily. My concern are systems with an I/O address space
(behind the IOMMU) distinct from the CPU address space. Iirc at
least Alpha is/was that way.

Jan



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