RE: [PATCH v8 3/5] iommufd: Add IOMMU_GET_HW_INFO

[Liu, Yi L]

> From: Nicolin Chen <nicolinc@xxxxxxxxxx>
> Sent: Friday, August 18, 2023 5:08 AM
> 
> Looks like Yi's latest code has not addressed these comments.

Yeah. Not yet. In progress to incorporate them. 😊

> 
> On Thu, Aug 17, 2023 at 07:31:42AM +0000, Tian, Kevin wrote:
> 
> > > From: Liu, Yi L <yi.l.liu@xxxxxxxxx>
> > > Sent: Wednesday, August 16, 2023 8:14 PM
> > >
> > > Under nested IOMMU translation, userspace owns the stage-1 translation
> > > table (e.g. the stage-1 page table of Intel VT-d or the context table of
> > > ARM SMMUv3, and etc.). Stage-1 translation tables are vendor specific, and
> > > need to be compatible with the underlying IOMMU hardware. Hence,
> > > userspace
> > > should know the IOMMU hardware capability before creating and
> > > configuring
> > > the stage-1 translation table to kernel.
> > >
> > > This adds IOMMU_GET_HW_INFO ioctl to query the IOMMU hardware
> > > information
> > > (a.k.a capability) for a given device. The returned data is vendor
> > > specific, userspace needs to decode it with the structure by the output
> > > @out_data_type field.
> >
> > "The format of the returned data is vendor specific and must be decoded
> > according to @out_data_type field".
> 
> Ack.
> 
> > > +int iommufd_get_hw_info(struct iommufd_ucmd *ucmd)
> > > +{
> > > +     struct iommu_hw_info *cmd = ucmd->cmd;
> > > +     void __user *user_ptr = u64_to_user_ptr(cmd->data_uptr);
> > > +     const struct iommu_ops *ops;
> > > +     struct iommufd_device *idev;
> > > +     unsigned int data_len;
> > > +     unsigned int copy_len;
> > > +     void *data = NULL;
> [..]
> > > +     } else {
> > > +             cmd->out_data_type = IOMMU_HW_INFO_TYPE_NONE;
> > > +             data_len = 0;
> > > +             data = NULL;
> >
> > data is already initialized as NULL.

Probably we can set data_len = 0 and the out_data_type to _NONE is
the top as well. Any preference?

> 
> Will drop.
> 
> > > +
> > > +     /*
> > > +      * We return the length the kernel supports so userspace may know
> > > what
> > > +      * the kernel capability is. It could be larger than the input buffer.
> > > +      */
> > > +     cmd->data_len = data_len;
> > > +
> > > +     rc = iommufd_ucmd_respond(ucmd, sizeof(*cmd));
> > > +out:
> >
> > out_free:
> >
> > > +     kfree(data);
> > > +err_put:
> >
> > out_put: (since this is also used in the success path)
> 
> Ack for both.
> 
> > > + * To capture an iommu type specific hardware information data,
> > > @data_uptr and
> > > + * its length @data_len must be provided. Trailing bytes will be zeroed if the
> > > + * user buffer is larger than the data that kernel has. Otherwise, kernel only
> > > + * fills the buffer using the given length in @data_len. If the ioctl succeeds,
> > > + * @data_len will be updated to the length that kernel actually supports,
> > > + * @out_data_type will be filled to decode the data filled in the buffer
> > > + * pointed by @data_uptr. Input @data_len == zero is allowed, no
> > > information
> > > + * data will be filled to user, but user space could get the
> > > iommu_hw_info_type
> > > + * filled in @out_data_type and the iommu hardware information data
> > > length
> > > + * supported by kernel filled in @data_len.
> >
> > I'd just keep "Input @data_len == zero is allowed" and remove all the
> > trailing words which just duplicate with the former context.
> 
> Will do.
> 
> > Reviewed-by: Kevin Tian <kevin.tian@xxxxxxxxx>
> 
> Adding this.
> 
> Thanks
> Nic