linux-kernel - Re: [PATCH v2 05/10] iommufd: Add replace support in iommufd_access_set

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Message-ID: <Y+Vcc6x2JVPwkl5+@nvidia.com>
Date:   Thu, 9 Feb 2023 16:49:55 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Nicolin Chen <nicolinc@...dia.com>
Cc:     "Tian, Kevin" <kevin.tian@...el.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "will@...nel.org" <will@...nel.org>,
        "robin.murphy@....com" <robin.murphy@....com>,
        "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
        "shuah@...nel.org" <shuah@...nel.org>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
        "baolu.lu@...ux.intel.com" <baolu.lu@...ux.intel.com>
Subject: Re: [PATCH v2 05/10] iommufd: Add replace support in
 iommufd_access_set_ioas()

On Thu, Feb 09, 2023 at 12:28:45PM -0800, Nicolin Chen wrote:
> On Thu, Feb 09, 2023 at 03:13:08AM +0000, Tian, Kevin wrote:
>  
> > > --- a/drivers/iommu/iommufd/device.c
> > > +++ b/drivers/iommu/iommufd/device.c
> > > @@ -509,11 +509,23 @@ int iommufd_access_set_ioas(struct
> > > iommufd_access *access, u32 ioas_id)
> > >               iommufd_ref_to_users(obj);
> > >       }
> > >
> > > +     /*
> > > +      * Set ioas to NULL to block any further iommufd_access_pin_pages().
> > > +      * iommufd_access_unpin_pages() can continue using access-
> > > >ioas_unpin.
> > > +      */
> > > +     access->ioas = NULL;
> > > +
> > >       if (cur_ioas) {
> > > +             if (new_ioas) {
> > > +                     mutex_unlock(&access->ioas_lock);
> > > +                     access->ops->unmap(access->data, 0, ULONG_MAX);
> > > +                     mutex_lock(&access->ioas_lock);
> > > +             }
> > 
> > why does above only apply to a valid new_ioas? this is the cleanup on
> > cur_ioas then required even when new_ioas=NULL.
>   
> Though it'd make sense to put it in the common path, our current
> detach routine doesn't call this unmap. If we do so, it'd become
> something new to the normal detach routine. Or does this mean the
> detach routine has been missing an unmap call so far?

By the time vfio_iommufd_emulated_unbind() is called the driver's
close_device() has already returned

At this point the driver should have removed all active pins.

We should not call back into the driver with unmap after its
close_device() returns.

However, this function is not on the close_device path so it should
always flush all existing mappings before attempting to change the
ioas to anything.

Jason