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

[Julien Grall <julien.grall@linaro.org>]

On 05/14/2014 02:08 PM, Jan Beulich wrote:
>>>> On 14.05.14 at 14:45, <julien.grall@linaro.org> wrote:
>> On 05/14/2014 10:25 AM, Jan Beulich wrote:
>>>>>> On 14.05.14 at 11:09, <julien.grall@linaro.org> 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.
> 
> Oh, right, I didn't pay attention to you passing a domain into the
> query.
> 
>> What about creating a callback:
>> iommu_has_feature(struct domain *d, enum iommu_feature)
>>
>> and let the driver handling the feature?
> 
> That sounds fine as long as you don't expect such to sit on any hot
> path (indirect calls are expensive not only on x86 I think).

Actually, it's called during an hot patch (every time a page is mapped
in the p2m).

I've already optimized a bit by only call the function one per batch of
mapping (as iommu TLB flush).

Regards,

-- 
Julien Grall