Re: [PATCH 08/12] iommufd/device: Report supported hwpt_types

[Nicolin Chen <nicolinc@nvidia.com>]

On Fri, Mar 10, 2023 at 07:39:00AM +0000, Liu, Yi L wrote:
> External email: Use caution opening links or attachments
> 
> 
> > From: Nicolin Chen <nicolinc@nvidia.com>
> > Sent: Friday, March 10, 2023 3:10 PM
> >
> > On Fri, Mar 10, 2023 at 11:30:04AM +0800, Baolu Lu wrote:
> > > External email: Use caution opening links or attachments
> > >
> > >
> > > On 3/9/23 4:09 PM, Yi Liu wrote:
> > > > This provides a way for userspace to probe the supported hwpt data
> > > > types by kernel. Currently, kernel only supports
> > IOMMU_HWPT_TYPE_DEFAULT,
> > > > new types would be added per vendor drivers' extension.
> > > >
> > > > Userspace that wants to allocate hw_pagetable with user data should
> > check
> > > > this. While for the allocation without user data, no need for it. It is
> > > > supported by default.
> > > >
> > > > Co-developed-by: Nicolin Chen <nicolinc@nvidia.com>
> > > > Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
> > > > Signed-off-by: Yi Liu <yi.l.liu@intel.com>
> > > > ---
> > > >   drivers/iommu/iommufd/device.c          |  1 +
> > > >   drivers/iommu/iommufd/hw_pagetable.c    | 18 +++++++++++++++---
> > > >   drivers/iommu/iommufd/iommufd_private.h |  2 ++
> > > >   drivers/iommu/iommufd/main.c            |  2 +-
> > > >   include/uapi/linux/iommufd.h            |  8 ++++++++
> > > >   5 files changed, 27 insertions(+), 4 deletions(-)
> > > >
> > > > diff --git a/drivers/iommu/iommufd/device.c
> > b/drivers/iommu/iommufd/device.c
> > > > index 19cd6df46c6a..0328071dcac1 100644
> > > > --- a/drivers/iommu/iommufd/device.c
> > > > +++ b/drivers/iommu/iommufd/device.c
> > > > @@ -322,6 +322,7 @@ int iommufd_device_get_hw_info(struct
> > iommufd_ucmd *ucmd)
> > > >
> > > >       cmd->out_data_type = ops->driver_type;
> > > >       cmd->data_len = length;
> > > > +     cmd->out_hwpt_type_bitmap =
> > iommufd_hwpt_type_bitmaps[ops->driver_type];
> > > >
> > > >       rc = iommufd_ucmd_respond(ucmd, sizeof(*cmd));
> > > >
> > > > diff --git a/drivers/iommu/iommufd/hw_pagetable.c
> > b/drivers/iommu/iommufd/hw_pagetable.c
> > > > index 67facca98de1..160712256c64 100644
> > > > --- a/drivers/iommu/iommufd/hw_pagetable.c
> > > > +++ b/drivers/iommu/iommufd/hw_pagetable.c
> > > > @@ -173,6 +173,14 @@ static const size_t
> > iommufd_hwpt_alloc_data_size[] = {
> > > >       [IOMMU_HWPT_TYPE_DEFAULT] = 0,
> > > >   };
> > > >
> > > > +/*
> > > > + * bitmaps of supported hwpt types of by underlying iommu, indexed
> > > > + * by ops->driver_type which is one of enum iommu_hw_info_type.
> > > > + */
> > > > +const u64 iommufd_hwpt_type_bitmaps[] =  {
> > > > +     [IOMMU_HW_INFO_TYPE_DEFAULT] =
> > BIT_ULL(IOMMU_HWPT_TYPE_DEFAULT),
> > > > +};
> > >
> > > I am a bit confused here. Why do you need this array? What I read is
> > > that you want to convert ops->driver_type to a bit position in
> > > cmd->out_hwpt_type_bitmap.
> > >
> > > Am I getting it right?
> > >
> > > If so, why not just
> > >        cmd->out_hwpt_type_bitmap = BIT_ULL(ops->driver_type);
> > >
> > > ?
> 
> The reason is for future extensions. If future usages need different types
> of user data to allocate hwpt,  it can define a new type and corresponding
> data structure. Such new usages may be using new vendor-specific page
> tables or vendor-agnostic usages like Re-use of the KVM page table in
> the IOMMU mentioned by IOMMUFD basic series.
> 
> https://lore.kernel.org/kvm/0-v6-a196d26f289e+11787-iommufd_jgg@nvidia.com/
> 
> > A driver_type would be IOMMUFD_HW_INFO_TYPExx. What's inside the
> > BIT_ULL is IOMMUFD_HWPT_TYPE_*. It seems to get a bit confusing
> > after several rounds of renaming though. And they do seem to be
> > a bit of duplications at the actual values, at least for now.
> 
> For now, vendor drivers only have one stage-1 page table format.
> But in the future, it may change per new page table format
> introduction and new usage.

Yea, that's what I thought too. Yet, I am wondering a bit if
it'd be better to have an ops->hwpt_type in the drivers, v.s.
maintaining a potentially big chunk of the array here.

Thanks
Nic