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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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.

Jan



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