Re: [PATCH v3 02/12] iommu: Add iommu_split_block interface

[Lu Baolu <baolu.lu@linux.intel.com>]

On 4/20/21 3:32 PM, Keqian Zhu wrote:
> Hi Baolu,
> 
> Cheers for the your quick reply.
> 
> On 2021/4/20 10:09, Lu Baolu wrote:
>> Hi Keqian,
>>
>> On 4/20/21 9:25 AM, Keqian Zhu wrote:
>>> Hi Baolu,
>>>
>>> On 2021/4/19 21:33, Lu Baolu wrote:
>>>> Hi Keqian,
>>>>
>>>> On 2021/4/19 17:32, Keqian Zhu wrote:
>>>>>>> +EXPORT_SYMBOL_GPL(iommu_split_block);
>>>>>> Do you really have any consumers of this interface other than the dirty
>>>>>> bit tracking? If not, I don't suggest to make this as a generic IOMMU
>>>>>> interface.
>>>>>>
>>>>>> There is an implicit requirement for such interfaces. The
>>>>>> iommu_map/unmap(iova, size) shouldn't be called at the same time.
>>>>>> Currently there's no such sanity check in the iommu core. A poorly
>>>>>> written driver could mess up the kernel by misusing this interface.
>>>>> Yes, I don't think up a scenario except dirty tracking.
>>>>>
>>>>> Indeed, we'd better not make them as a generic interface.
>>>>>
>>>>> Do you have any suggestion that underlying iommu drivers can share these code but
>>>>> not make it as a generic iommu interface?
>>>>>
>>>>> I have a not so good idea. Make the "split" interfaces as a static function, and
>>>>> transfer the function pointer to start_dirty_log. But it looks weird and inflexible.
>>>>
>>>> I understand splitting/merging super pages is an optimization, but not a
>>>> functional requirement. So is it possible to let the vendor iommu driver
>>>> decide whether splitting super pages when starting dirty bit tracking
>>>> and the opposite operation during when stopping it? The requirement for
>>> Right. If I understand you correct, actually that is what this series does.
>>
>> I mean to say no generic APIs, jut do it by the iommu subsystem itself.
>> It's totally transparent to the upper level, just like what map() does.
>> The upper layer doesn't care about either super page or small page is
>> in use when do a mapping, right?
>>
>> If you want to consolidate some code, how about putting them in
>> start/stop_tracking()?
> 
> Yep, this reminds me. What we want to reuse is the logic of "chunk by chunk" in split().
> We can implement switch_dirty_log to be "chunk by chunk" too (just the same as sync/clear),
> then the vendor iommu driver can invoke it's own private implementation of split().
> So we can completely remove split() in the IOMMU core layer.
> 
> example code logic
> 
> iommu.c:
> switch_dirty_log(big range) {
>      for_each_iommu_page(big range) {
>            ops->switch_dirty_log(iommu_pgsize)
>      }
> }
> 
> vendor iommu driver:
> switch_dirty_log(iommu_pgsize) {
> 
>      if (enable) {
>          ops->split_block(iommu_pgsize)
>          /* And other actions, such as enable hardware capability */
>      } else {
>          for_each_continuous_physical_address(iommu_pgsize)
>              ops->merge_page()
>      }
> }
> 
> Besides, vendor iommu driver can invoke split() in clear_dirty_log instead of in switch_dirty_log.
> The benefit is that we usually clear dirty log gradually during dirty tracking, then we can split
> large page mapping gradually, which speedup start_dirty_log and make less side effect on DMA performance.
> 
> Does it looks good for you?

Yes. It's clearer now.

> 
> Thanks,
> Keqian
> 

Best regards,
baolu

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