linux-kernel - RE: [PATCH 05/12] iommufd/hw_pagetable: Do not populate user-managed hw

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Date:   Thu, 23 Mar 2023 08:28:39 +0000
From:   "Liu, Yi L" <yi.l.liu@...el.com>
To:     Nicolin Chen <nicolinc@...dia.com>
CC:     Baolu Lu <baolu.lu@...ux.intel.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
        "jgg@...dia.com" <jgg@...dia.com>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        "robin.murphy@....com" <robin.murphy@....com>,
        "cohuck@...hat.com" <cohuck@...hat.com>,
        "eric.auger@...hat.com" <eric.auger@...hat.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "mjrosato@...ux.ibm.com" <mjrosato@...ux.ibm.com>,
        "chao.p.peng@...ux.intel.com" <chao.p.peng@...ux.intel.com>,
        "yi.y.sun@...ux.intel.com" <yi.y.sun@...ux.intel.com>,
        "peterx@...hat.com" <peterx@...hat.com>,
        "jasowang@...hat.com" <jasowang@...hat.com>,
        "shameerali.kolothum.thodi@...wei.com" 
        <shameerali.kolothum.thodi@...wei.com>,
        "lulu@...hat.com" <lulu@...hat.com>,
        "suravee.suthikulpanit@....com" <suravee.suthikulpanit@....com>,
        "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>
Subject: RE: [PATCH 05/12] iommufd/hw_pagetable: Do not populate user-managed
 hw_pagetables

> From: Nicolin Chen <nicolinc@...dia.com>
> Sent: Thursday, March 23, 2023 4:12 PM
> 
> On Thu, Mar 23, 2023 at 08:06:26AM +0000, Liu, Yi L wrote:
> > External email: Use caution opening links or attachments
> >
> >
> > > From: Nicolin Chen <nicolinc@...dia.com>
> > > Sent: Friday, March 10, 2023 2:51 PM
> >  >
> > > 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@...dia.com>
> > > > >
> > > > > 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@...dia.com>
> > > > > Signed-off-by: Yi Liu <yi.l.liu@...el.com>
> > > > > ---
> > > > >   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.
> >
> > I think I was convinced with the reason that this helper may be
> > called by allocation for both standalone s2 hwpt and the nested
> > hwpt. So my change was not enough. Yours covers both cases.
> >
> > > But,
> > > the link between the parent hwpt and the IOAS happened inside
> > > the allocation function now, not attach() any more.
> >
> > Not quite get. This helper is also called in the allocation path. Is
> > it? Anyhow, with Jason's comment, this helper may be reworked.
> > We can sync later on the next version.
> 
> We previously had this link_ioas() in attach() routine so we
> needed to make sure hwpt->parent got populated, because the
> device could be attached to an S1 HWPT directly. But now this
> is in the alloc() routine, so by the time an S1 HWPT is being
> allocated, an S2 HWPT must be allocated first and populated
> already.

Aha, yes. 😊