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

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> Sent: 05 September 2018 08:12
> 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 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.
> 

Personally I think 'bus address' is commonly enough used term for addresses 
used by devices for DMA. Indeed we have already 'dev_bus_addr' in the grant map 
and unmap hypercalls. So baddr and bfn seem like ok terms to me. It's also not 
impossible to rename these later if they prove problematic.

  Paul

> Jan
> 


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