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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 14.05.19 at 18:19, <Paul.Durrant@xxxxxxxxxx> 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.
>>
> 
> 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.

Different quirks could likely be handled with a global ops instance.
The indirect call overhead elimination alone will imo make it
undesirable to switch to a non-global-ops model on x86, unless
there's a strong reason (like truly different IOMMUs in a single
system).

Jan



_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel