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

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----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.

  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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