Re: [Xen-devel] [PATCH 3/5] iommu: move iommu_get_ops() into common code

[Julien Grall <julien.grall@xxxxxxx>]

Hi,

On 5/14/19 5:19 PM, Paul Durrant wrote:
> > -----Original Message-----
> > From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> > Sent: 13 May 2019 09:11
> > To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
> > Cc: Brian Woods <brian.woods@xxxxxxx>; Suravee Suthikulpanit 
> > <suravee.suthikulpanit@xxxxxxx>; Julien
> > Grall <julien.grall@xxxxxxx>; Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>; Roger 
> > Pau Monne
> > <roger.pau@xxxxxxxxxx>; Wei Liu <wei.liu2@xxxxxxxxxx>; Kevin Tian 
> > <kevin.tian@xxxxxxxxx>; Stefano
> > Stabellini <sstabellini@xxxxxxxxxx>; xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
> > Subject: Re: [PATCH 3/5] iommu: move iommu_get_ops() into common code
> >
> > > > > On 08.05.19 at 15:24, <paul.durrant@xxxxxxxxxx> wrote:
> > > Currently x86 and ARM differ in their implementation for no good reason.
> > > This patch moves the ARM variant of iommu_get/set_ops() helpers into
> > > common code and modifies them so they deal with the __initconstrel
> > > ops structures used by the x86 IOMMU vendor implementations (adding
> > > __initconstrel to the SMMU code to bring it in line). Consequently, a lack
> > > of init() method is now taken to mean uninitialized iommu_ops. Also, the
> > > printk warning in iommu_set_ops() now becomes an ASSERT.
> > When having submitted the indirect call overhead reduction series
> > including IOMMU changes for the first time, I was told that the Arm
> > folks would like to retain the ability to eventually support
> > heterogeneous IOMMUs (and hence I shouldn't provide patching
> > infrastructure there). A single global iommu_[gs]et_ops() is sort of
> > getting in the way of this as well, I think, and hence I'm not sure it
> > is a desirable step to make this so far Arm-specific arrangement
> > the general model. At least it would further complicate Arm side
> > changes towards that (mid / long term?) goal.
That's correct, it is a mid / long term plan.

> >
> Ok. Do you have any more information on what such an architecture would look 
> like? I guess it is also conceivable that an x86 architecture might have 
> slightly different IOMMU implementations (or at least quirks) for different PCI 
> segments. So perhaps a global ops structure is not a good idea in the long run.
I can see two uses cases:
    1) Finding the IOMMU associated to a device
    2) Applying an operation (i.e domain creation/destruction, 
map/unmap) on all the IOMMU
Actually, we already have similar concept within the SMMU driver because 
a platform may contain multiple SMMUs.
Any generic interface would actually be quite beneficial as we could 
simplify a lot the driver and avoid duplicating the logic in all the new 
Arm drivers.
Cheers,

--
Julien Grall

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