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

[Liu, Yi L]

> From: Nicolin Chen <nicolinc@xxxxxxxxxx>
> 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@xxxxxxxxxx>
> > > Signed-off-by: Nicolin Chen <nicolinc@xxxxxxxxxx>
> > > Signed-off-by: Yi Liu <yi.l.liu@xxxxxxxxx>
> > > ---
> > >   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@xxxxxxxxxx/
 
> 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.

Regards,
Yi Liu