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

From: "Jan Beulich" <JBeulich@suse.com>
To: Paul Durrant <paul.durrant@citrix.com>
Cc: Kevin Tian <kevin.tian@intel.com>,
	Stefano Stabellini <sstabellini@kernel.org>,
	Wei Liu <wei.liu2@citrix.com>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	Julien Grall <julien.grall@arm.com>,
	Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>,
	xen-devel <xen-devel@lists.xenproject.org>,
	Brian Woods <brian.woods@amd.com>,
	Roger Pau Monne <roger.pau@citrix.com>
Subject: Re: [PATCH 3/5] iommu: move iommu_get_ops() into common code
Date: Wed, 15 May 2019 00:32:51 -0600	[thread overview]
Message-ID: <5CDBB293020000780022F118@prv1-mh.provo.novell.com> (raw)
In-Reply-To: <246a87e654194e5082852b63853415d6@AMSPEX02CL03.citrite.net>

>>> On 14.05.19 at 18:19, <Paul.Durrant@citrix.com> wrote:
>>  -----Original Message-----
>> From: Jan Beulich [mailto:JBeulich@suse.com]
>> Sent: 13 May 2019 09:11
>> To: Paul Durrant <Paul.Durrant@citrix.com>
>> Cc: Brian Woods <brian.woods@amd.com>; Suravee Suthikulpanit 
> <suravee.suthikulpanit@amd.com>; Julien
>> Grall <julien.grall@arm.com>; Andrew Cooper <Andrew.Cooper3@citrix.com>; Roger 
> Pau Monne
>> <roger.pau@citrix.com>; Wei Liu <wei.liu2@citrix.com>; Kevin Tian 
> <kevin.tian@intel.com>; Stefano
>> Stabellini <sstabellini@kernel.org>; xen-devel <xen-devel@lists.xenproject.org>
>> Subject: Re: [PATCH 3/5] iommu: move iommu_get_ops() into common code
>> 
>> >>> On 08.05.19 at 15:24, <paul.durrant@citrix.com> 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@lists.xenproject.org
https://lists.xenproject.org/mailman/listinfo/xen-devel