> -----Original Message-----
> From: Jan Beulich [mailto:jbeul...@suse.com]
> Sent: 13 May 2019 09:11
> To: Paul Durrant <paul.durr...@citrix.com>
> Cc: Brian Woods <brian.wo...@amd.com>; Suravee Suthikulpanit
> <suravee.suthikulpa...@amd.com>; Julien
> Grall <julien.gr...@arm.com>; Andrew Cooper <andrew.coop...@citrix.com>;
> Roger Pau Monne
> <roger....@citrix.com>; Wei Liu <wei.l...@citrix.com>; Kevin Tian
> <kevin.t...@intel.com>; Stefano
> Stabellini <sstabell...@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.durr...@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.
Paul
> > --- a/xen/drivers/passthrough/iommu.c
> > +++ b/xen/drivers/passthrough/iommu.c
> > @@ -21,6 +21,21 @@
> > #include <xen/keyhandler.h>
> > #include <xsm/xsm.h>
> >
> > +static struct iommu_ops __read_mostly iommu_ops;
> > +
> > +const struct iommu_ops *iommu_get_ops(void)
> > +{
> > + return &iommu_ops;
> > +}
> > +
> > +void __init iommu_set_ops(const struct iommu_ops *ops)
> > +{
> > + BUG_ON(!ops);
> > +
> > + ASSERT(!iommu_ops.init || iommu_ops.init == ops->init);
> > + iommu_ops = *ops;
> > +}
>
> I realize that you merely move (and slightly re-arrange) what has
> been there, but now that I look at it again I think ops->init should
> also be verified to be non-NULL, or else installing such a set of
> hooks would effectively revert back to the "no hooks yet" state.
>
> > @@ -33,11 +32,7 @@ int __init iommu_hardware_setup(void)
> > if ( !iommu_init_ops )
> > return -ENODEV;
> >
> > - if ( !iommu_ops.init )
> > - iommu_ops = *iommu_init_ops->ops;
> > - else
> > - /* x2apic setup may have previously initialised the struct. */
> > - ASSERT(iommu_ops.init == iommu_init_ops->ops->init);
> > + iommu_set_ops(iommu_init_ops->ops);
>
> I was specifically asked to add the comment that you get rid of.
> While mentioning x2APIC in common code may no be appropriate,
> I'm sure this could be worded in a more general way and attached
> to the moved check.
>
> Jan
>
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