Re: [Xen-devel] [PATCH v5 12/14] xen/arm: p2m: Clean cache PT when the IOMMU doesn't support coherent walk

[Julien Grall <julien.grall@xxxxxxxxxx>]

On 05/14/2014 10:25 AM, Jan Beulich wrote:
>>>> On 14.05.14 at 11:09, <julien.grall@xxxxxxxxxx> wrote:
>>>> @@ -139,6 +147,7 @@ struct iommu_ops {
>>>>       void (*iotlb_flush)(struct domain *d, unsigned long gfn, unsigned 
>>>> int page_count);
>>>>       void (*iotlb_flush_all)(struct domain *d);
>>>>       void (*dump_p2m_table)(struct domain *d);
>>>> +    uint32_t (*features)(struct domain *d);
>>>
>>> I think I said this on an earlier round already - for future extensibility
>>> this should return "const unsigned long *", and get accessed by the
>>> wrapper function using test_bit(). Or even better without an accessor
>>> function at all, just directly having a "const unsigned long *" field here.
>>> Unless of course the backend implementation - which isn't part of this
>>> patch - would have difficulty setting up a suitable bitfield during (early)
>>> initialization.
>>
>> The SMMU drivers handle multiple SMMUs. Each SMMU can have different 
>> specifications (e.g coherent walk support or not).
>>
>> As each domain doesn't use all SMMUs, we might be able to avoid flushing 
>> PT on some of them. That's why I've choose to use a callback with the 
>> domain in parameter.
>>
>> I don't like the solution which return "unsigned long *" because we are 
>> assuming the driver will always a valid pointer (for instance with 2 
>> unsigned long), even if he doesn't need it.
> 
> In that case simply make this a bitmap embedded in struct iommu_ops,
> which the driver has to populate suitably early. That way the individual
> drivers don't need to care about the eventually growing size.

That doesn't really help me. Because a same platform can have a mix of
SMMU support coherent walk or not.

For optimization it would be better to have a per-domain feature.

What about creating a callback:
iommu_has_feature(struct domain *d, enum iommu_feature)

and let the driver handling the feature?

Regards,

-- 
Julien Grall

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