Re: [PATCH 05/12] iommufd/hw_pagetable: Do not populate user-managed hw_pagetables

[Baolu Lu]

On 2023/3/10 14:50, Nicolin Chen wrote:
> On Fri, Mar 10, 2023 at 10:25:10AM +0800, Baolu Lu wrote:
> > External email: Use caution opening links or attachments
> >
> >
> > On 3/9/23 4:09 PM, Yi Liu wrote:
> > > From: Nicolin Chen <nicolinc@xxxxxxxxxx>
> > >
> > > A user-managed hw_pagetable does not need to get populated, since it is
> > > managed by a guest OS. Move the iopt_table_add_domain and list_add_tail
> > > calls into a helper, where the hwpt pointer will be redirected to its
> > > hwpt->parent if it's available.
> > >
> > > Signed-off-by: Nicolin Chen <nicolinc@xxxxxxxxxx>
> > > Signed-off-by: Yi Liu <yi.l.liu@xxxxxxxxx>
> > > ---
> > >    drivers/iommu/iommufd/hw_pagetable.c | 20 ++++++++++++++++++--
> > >    1 file changed, 18 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/drivers/iommu/iommufd/hw_pagetable.c b/drivers/iommu/iommufd/hw_pagetable.c
> > > index 16e92a1c150b..6e45ec0a66fa 100644
> > > --- a/drivers/iommu/iommufd/hw_pagetable.c
> > > +++ b/drivers/iommu/iommufd/hw_pagetable.c
> > > @@ -43,6 +43,23 @@ int iommufd_hw_pagetable_enforce_cc(struct iommufd_hw_pagetable *hwpt)
> > >        return 0;
> > >    }
> > >
> > > +static int iommufd_hw_pagetable_link_ioas(struct iommufd_hw_pagetable *hwpt)
> > > +{
> > > +     int rc;
> > > +
> > > +     if (hwpt->parent)
> > > +             hwpt = hwpt->parent;
> > > +
> > > +     if (!list_empty(&hwpt->hwpt_item))
> > > +             return 0;
> >
> > What is above check for? Is it "the hwpt has already been inserted into
> > the hwpt list of its ioas in another place"?
> >
> > If so, is it possible that hwpt will be deleted from the list even when
> > this user hwpt is still linked to the ioas?
>
> It means that the hwpt is already linked to the ioas. And the
> hwpt_item can be only empty after a destroy().
>
> With that being said, after I think it through, perhaps Yi's
> previous change removing it might be better. So, it could be:
>
> -------------------------------------------------------------------------------
> +	/*
> +	 * Only a parent hwpt needs to be linked to the IOAS. And a hwpt->parent
> +	 * must be linked to the IOAS already, when it's being allocated.
> +	 */
>   	if (hwpt->parent)
> -		hwpt = hwpt->parent;
> -
> -	if (!list_empty(&hwpt->hwpt_item))
>   		return 0;
>   
> -------------------------------------------------------------------------------
>
> I was concerned about the case where a device gets attached to
> the nested hwpt without staging at the parent hwpt first. But,
> the link between the parent hwpt and the IOAS happened inside
> the allocation function now, not attach() any more.

Yes, it's clearer.

Best regards,
baolu