linux-kernel - Re: [PATCH v3 02/12] iommu: Add iommu_split

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Message-ID: <3c34baf1-6a57-5666-38a2-0c9d6188b8b8@linux.intel.com>
Date:   Tue, 20 Apr 2021 10:09:08 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     Keqian Zhu <zhukeqian1@...wei.com>, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        iommu@...ts.linux-foundation.org,
        Robin Murphy <robin.murphy@....com>,
        Will Deacon <will@...nel.org>, Joerg Roedel <joro@...tes.org>,
        Yi Sun <yi.y.sun@...ux.intel.com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Tian Kevin <kevin.tian@...el.com>
Cc:     baolu.lu@...ux.intel.com,
        Alex Williamson <alex.williamson@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Kirti Wankhede <kwankhede@...dia.com>,
        wanghaibin.wang@...wei.com, jiangkunkun@...wei.com,
        yuzenghui@...wei.com, lushenming@...wei.com
Subject: Re: [PATCH v3 02/12] iommu: Add iommu_split_block interface

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()?

Best regards,
baolu

> We realized split/merge in IOMMU core layer, but don't force vendor driver to use it.
> 
> The problem is that when we expose these interfaces to vendor IOMMU driver, will also
> expose them to upper driver.
> 
>> upper layer is that starting/stopping dirty bit tracking and
>> mapping/unmapping are mutually exclusive.
> OK, I will explicitly add the hints. Thanks.
> 
> Thanks,
> Keqian
>>
>>>
>>> On the other hand, if a driver calls map/unmap with split/merge at the same time,
>>> it's a bug of driver, it should follow the rule.
>>>
>>
>> Best regards,
>> baolu
>> .
>>