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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 05.09.18 at 08:56, <kevin.tian@xxxxxxxxx> wrote:
>>  From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
>> Sent: Wednesday, September 5, 2018 2:49 PM
>> 
>> >>> On 05.09.18 at 02:42, <kevin.tian@xxxxxxxxx> wrote:
>> >>  From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
>> >> Sent: Tuesday, September 4, 2018 5:08 PM
>> >>
>> >> >>> 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.
>> >>
>> >
>> > Then Paul please documents clearly that this bus address refers to
>> > the input side of IOMMU. :-)
>> 
>> But when reading code you can't always go back to look at the one
>> place where its meaning is documented. Hence my desire for a name
>> which properly conveys the meaning.
>> 
> 
> Then possibly go back to DFN, but take 'D' as DMA instead of device?

How would "DMA" be any better than "bus"? Whose view it is then still
is unclear.

Jan



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