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

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Date:   Mon, 9 Oct 2023 13:13:45 +0800
From:   Yan Zhao <yan.y.zhao@...el.com>
To:     Yi Liu <yi.l.liu@...el.com>
CC:     <joro@...tes.org>, <alex.williamson@...hat.com>, <jgg@...dia.com>,
        <kevin.tian@...el.com>, <robin.murphy@....com>,
        <baolu.lu@...ux.intel.com>, <cohuck@...hat.com>,
        <eric.auger@...hat.com>, <nicolinc@...dia.com>,
        <kvm@...r.kernel.org>, <mjrosato@...ux.ibm.com>,
        <chao.p.peng@...ux.intel.com>, <yi.y.sun@...ux.intel.com>,
        <peterx@...hat.com>, <jasowang@...hat.com>,
        <shameerali.kolothum.thodi@...wei.com>, <lulu@...hat.com>,
        <suravee.suthikulpanit@....com>, <iommu@...ts.linux.dev>,
        <linux-kernel@...r.kernel.org>, <linux-kselftest@...r.kernel.org>,
        <zhenzhong.duan@...el.com>, <joao.m.martins@...cle.com>
Subject: Re: [PATCH v4 03/17] iommufd: Unite all kernel-managed members into
 a struct

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@...dia.com>
> > > 
> > > 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@...dia.com>
> > > Signed-off-by: Yi Liu <yi.l.liu@...el.com>
> > > ---
> > >   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.
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.

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