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

[Keqian Zhu]

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?

Thanks,
Keqian