Re: [PATCH v4 03/17] iommufd: Unite all kernel-managed members into a struct

[Yi Liu]

On 2023/10/9 13:13, Yan Zhao wrote:
> On Mon, Oct 09, 2023 at 12:13:52PM +0800, Yi Liu wrote:
> > On 2023/10/7 18:08, Yan Zhao wrote:
> > > On Thu, Sep 21, 2023 at 12:51:24AM -0700, Yi Liu wrote:
> > > > From: Nicolin Chen <nicolinc@xxxxxxxxxx>
> > > >
> > > > The struct iommufd_hw_pagetable has been representing a kernel-managed
> > > > HWPT, yet soon will be reused to represent a user-managed HWPT. These
> > > > two types of HWPTs has the same IOMMUFD object type and an iommu_domain
> > > > object, but have quite different attributes/members.
> > > >
> > > > Add a union in struct iommufd_hw_pagetable and group all the existing
> > > > kernel-managed members. One of the following patches will add another
> > > > struct for user-managed members.
> > > >
> > > > Signed-off-by: Nicolin Chen <nicolinc@xxxxxxxxxx>
> > > > Signed-off-by: Yi Liu <yi.l.liu@xxxxxxxxx>
> > > > ---
> > > >    drivers/iommu/iommufd/iommufd_private.h | 17 +++++++++++------
> > > >    1 file changed, 11 insertions(+), 6 deletions(-)
> > > >
> > > > diff --git a/drivers/iommu/iommufd/iommufd_private.h b/drivers/iommu/iommufd/iommufd_private.h
> > > > index 3064997a0181..947a797536e3 100644
> > > > --- a/drivers/iommu/iommufd/iommufd_private.h
> > > > +++ b/drivers/iommu/iommufd/iommufd_private.h
> > > > @@ -231,13 +231,18 @@ int iommufd_vfio_ioas(struct iommufd_ucmd *ucmd);
> > > >     */
> > > >    struct iommufd_hw_pagetable {
> > > >    	struct iommufd_object obj;
> > > > -	struct iommufd_ioas *ioas;
> > > >    	struct iommu_domain *domain;
> > > > -	bool auto_domain : 1;
> > > > -	bool enforce_cache_coherency : 1;
> > > > -	bool msi_cookie : 1;
> > > > -	/* Head at iommufd_ioas::hwpt_list */
> > > > -	struct list_head hwpt_item;
> > > > +
> > > > +	union {
> > > > +		struct { /* kernel-managed */
> > > > +			struct iommufd_ioas *ioas;
> > > > +			bool auto_domain : 1;
> > > Will iommufd_hw_pagetable_put() also be called on non-kernel-managed domain?
> >
> > yes.
> >
> > > If yes, hwpt->user_managed needs to be checked in iommufd_hw_pagetable_put(),
> > > (e.g. as below).
> > > Otherwise, this union will lead to hwpt->ioas and hwpt->auto_domain still being
> > > accessible though invalid.
> >
> > not quite get this sentence.
> I mean with this union, hwpt->auto_domain or hwpt->ioas should only be accessed if and
> only if hwpt type is kernel-managed.

ok, I get this part. just not sure about the missing words in your prior 
comment.

> So, any unconditional access, as in iommufd_hw_pagetable_put() pasted below, is buggy.
>
> Therefore, do you think it's better to add a filed like "bool kernel_managed : 1",
> and access the union fields under  /* kernel-managed */ only when hwpt->kernel_managed
> is true.
>
>
> > >    static inline void iommufd_hw_pagetable_put(struct iommufd_ctx *ictx,
> > >                                               struct iommufd_hw_pagetable *hwpt)
> > >    {
> > > -       lockdep_assert_not_held(&hwpt->ioas->mutex);
> > > -       if (hwpt->auto_domain)
> > > +       if (!hwpt->user_managed)
> > > +               lockdep_assert_not_held(&hwpt->ioas->mutex);
> >
> > this is true. this assert is not needed when hwpt is not kernel managed domain.
> >
> > > +
> > > +       if (!hwpt->user_managed && hwpt->auto_domain)
> >
> > actually, checking auto_domain is more precise. There is hwpt which is
> > neither user managed nor auto.
>
> auto_domain is under union fields marked with kernel-managed only.
> Access it without type checking is invalid.

I see. yes, should check user_managed as well. :)

> struct iommufd_hw_pagetable {
>          struct iommufd_object obj;
>          struct iommu_domain *domain;
>
>          void (*abort)(struct iommufd_object *obj);
>          void (*destroy)(struct iommufd_object *obj);
>
>          bool user_managed : 1;
>          union {
>                  struct { /* user-managed */
>                          struct iommufd_hw_pagetable *parent;
>                  };
>                  struct { /* kernel-managed */
>                          struct iommufd_ioas *ioas;
>                          struct mutex mutex;
>                          bool auto_domain : 1;
>                          bool enforce_cache_coherency : 1;
>                          bool msi_cookie : 1;
>                          bool nest_parent : 1;
>                          /* Head at iommufd_ioas::hwpt_list */
>                          struct list_head hwpt_item;
>                  };
>          };
> };
>
> > >                   iommufd_object_deref_user(ictx, &hwpt->obj);
> > >           else
> > >                   refcount_dec(&hwpt->obj.users);
> > > }
> > >
> > > > +			bool enforce_cache_coherency : 1;
> > > > +			bool msi_cookie : 1;
> > > > +			/* Head at iommufd_ioas::hwpt_list */
> > > > +			struct list_head hwpt_item;
> > > > +		};
> > > > +	};
> > > >    };
> > > >    struct iommufd_hw_pagetable *
> > > > --
> > > > 2.34.1
> >
> > --
> > Regards,
> > Yi Liu

--
Regards,
Yi Liu