Re: [PATCH v2 04/11] iommu/arm-smmu-v3: Split block descriptor when start dirty log

[Yi Sun]

On 21-03-16 19:39:47, Keqian Zhu wrote:
> Hi Yi,
> 
> On 2021/3/16 17:17, Yi Sun wrote:
> > On 21-03-10 17:06:07, Keqian Zhu wrote:
> >> From: jiangkunkun <jiangkunkun@xxxxxxxxxx>
> >>
> >> Block descriptor is not a proper granule for dirty log tracking.
> >> Take an extreme example, if DMA writes one byte, under 1G mapping,
> >> the dirty amount reported to userspace is 1G, but under 4K mapping,
> >> the dirty amount is just 4K.
> >>
> >> This adds a new interface named start_dirty_log in iommu layer and
> >> arm smmuv3 implements it, which splits block descriptor to an span
> >> of page descriptors. Other types of IOMMU will perform architecture
> >> specific actions to start dirty log.
> >>
> >> To allow code reuse, the split_block operation is realized as an
> >> iommu_ops too. We flush all iotlbs after the whole procedure is
> >> completed to ease the pressure of iommu, as we will hanle a huge
> >> range of mapping in general.
> >>
> >> Spliting block does not simultaneously work with other pgtable ops,
> >> as the only designed user is vfio, which always hold a lock, so race
> >> condition is not considered in the pgtable ops.
> >>
> >> Co-developed-by: Keqian Zhu <zhukeqian1@xxxxxxxxxx>
> >> Signed-off-by: Kunkun Jiang <jiangkunkun@xxxxxxxxxx>
> >> ---
> >>
> >> changelog:
> >>
> >> v2:
> >>  - Change the return type of split_block(). size_t -> int.
> >>  - Change commit message to properly describe race condition. (Robin)
> >>  - Change commit message to properly describe the need of split block.
> >>  - Add a new interface named start_dirty_log(). (Sun Yi)
> >>  - Change commit message to explain the realtionship of split_block() and start_dirty_log().
> >>
> >> ---
> >>  drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c |  52 +++++++++
> >>  drivers/iommu/io-pgtable-arm.c              | 122 ++++++++++++++++++++
> >>  drivers/iommu/iommu.c                       |  48 ++++++++
> >>  include/linux/io-pgtable.h                  |   2 +
> >>  include/linux/iommu.h                       |  24 ++++
> >>  5 files changed, 248 insertions(+)
> >>
> > Could you please split iommu common interface to a separate patch?
> > This may make review and comments easier.
> Yup, good suggestion.
> 
> > 
> > IMHO, I think the start/stop interfaces could be merged into one, e.g:
> >     int iommu_domain_set_hwdbm(struct iommu_domain *domain, bool enable,
> >                                unsigned long iova, size_t size,
> >                                int prot);
> Looks good, this reduces some code. but I have a concern that this causes loss of flexibility,
> as we must pass same arguments when start|stop dirty log. What's your opinion about this?
> 
Per current design, start/stop interfaces have similar arguments. So I
think it is ok for now. For future extension, we may think to define a
structure to pass these arguments.

> > 
> > Same comments to patch 5.
> OK. Thanks.
> 
> > 
> > BRs,
> > Yi Sun
> > 
> >> -- 
> >> 2.19.1
> > .
> Thanks,
> Keqian

BRs,
Yi Sun