Re: [XEN][PATCH v7 09/19] xen/iommu: Move spin_lock from iommu_dt_device_is_assigned to caller

[Vikram Garhwal <vikram.garhwal@xxxxxxx>]

On Mon, Jun 05, 2023 at 08:19:38PM +0100, Julien Grall wrote:
> Hi,
> 
> On 02/06/2023 01:48, Vikram Garhwal wrote:
> > Rename iommu_dt_device_is_assigned() to 
> > iommu_dt_device_is_assigned_locked().
> > Remove static type so this can also be used by SMMU drivers to check if the
> > device is being used before removing.
> > 
> > Moving spin_lock to caller was done to prevent the concurrent access to
> > iommu_dt_device_is_assigned while doing add/remove/assign/deassign.
> 
> Can you explain if you are trying to resolve an existing bug, or this is
> something that will be necessary in a follow-up patch?
Updated for v8.
> 
> > 
> > Signed-off-by: Vikram Garhwal <vikram.garhwal@xxxxxxx>
> > 
> > ---
> > Changes from v6:
> >      Created a private header and moved iommu_dt_device_is_assigned() to 
> > header.
> > ---
> >   xen/drivers/passthrough/device_tree.c | 20 ++++++++++++++++----
> >   xen/include/xen/iommu-private.h       | 27 +++++++++++++++++++++++++++
> >   2 files changed, 43 insertions(+), 4 deletions(-)
> >   create mode 100644 xen/include/xen/iommu-private.h
> > 
> > diff --git a/xen/drivers/passthrough/device_tree.c 
> > b/xen/drivers/passthrough/device_tree.c
> > index 1c32d7b50c..52e370db01 100644
> > --- a/xen/drivers/passthrough/device_tree.c
> > +++ b/xen/drivers/passthrough/device_tree.c
> > @@ -18,6 +18,7 @@
> >   #include <xen/device_tree.h>
> >   #include <xen/guest_access.h>
> >   #include <xen/iommu.h>
> > +#include <xen/iommu-private.h>
> >   #include <xen/lib.h>
> >   #include <xen/sched.h>
> >   #include <xsm/xsm.h>
> > @@ -83,16 +84,14 @@ fail:
> >       return rc;
> >   }
> > -static bool_t iommu_dt_device_is_assigned(const struct dt_device_node *dev)
> > +bool_t iommu_dt_device_is_assigned_locked(const struct dt_device_node *dev)
> >   {
> 
> Please add an ASSERT() checking the lock is taken.
> 
Done in v8.
> >       bool_t assigned = 0;
> >       if ( !dt_device_is_protected(dev) )
> >           return 0;
> > -    spin_lock(&dtdevs_lock);
> >       assigned = !list_empty(&dev->domain_list);
> > -    spin_unlock(&dtdevs_lock);
> >       return assigned;
> >   }
> > @@ -213,27 +212,40 @@ int iommu_do_dt_domctl(struct xen_domctl *domctl, 
> > struct domain *d,
> >           if ( (d && d->is_dying) || domctl->u.assign_device.flags )
> >               break;
> > +        spin_lock(&dtdevs_lock);
> > +
> 
> 'dtdevs_lock' was intended to protect modification related to any IOMMU
> change. But here...
> 
> >           ret = dt_find_node_by_gpath(domctl->u.assign_device.u.dt.path,
> >                                       domctl->u.assign_device.u.dt.size,
> >                                       &dev);
> 
> ... you also include "dt_find_node_by_gpath". Can you explain why and add a
> comment on top of 'dtdevs_lock' to explain what it is intended use?
I have added a comment in v8. There was a comment in v3:
"ensure that the "dev" doesn't disappear between the time we look it up". So,
i moved the lock here and for dt_host the lock is added in follow-up patch:
"common/device_tree: Add rwlock for dt_host". So, this all will happen with
dtdevs_lock and dt_host_lock.
> 
> >           if ( ret )
> > +        {
> > +            spin_unlock(&dtdevs_lock);
> >               break;
> > +        }
> >           ret = xsm_assign_dtdevice(XSM_HOOK, d, dt_node_full_name(dev));
> >           if ( ret )
> > +        {
> > +            spin_unlock(&dtdevs_lock);
> >               break;
> > +        }
> >           if ( domctl->cmd == XEN_DOMCTL_test_assign_device )
> >           {
> > -            if ( iommu_dt_device_is_assigned(dev) )
> > +
> > +            if ( iommu_dt_device_is_assigned_locked(dev) )
> >               {
> >                   printk(XENLOG_G_ERR "%s already assigned.\n",
> >                          dt_node_full_name(dev));
> >                   ret = -EINVAL;
> >               }
> > +
> > +            spin_unlock(&dtdevs_lock);
> >               break;
> >           }
> > +        spin_unlock(&dtdevs_lock);
> > +
> >           if ( d == dom_io )
> >               return -EINVAL;
> > diff --git a/xen/include/xen/iommu-private.h 
> > b/xen/include/xen/iommu-private.h
> > new file mode 100644
> > index 0000000000..5615decaff
> > --- /dev/null
> > +++ b/xen/include/xen/iommu-private.h
> > @@ -0,0 +1,27 @@
> > +/* SPDX-License-Identifier: GPL-2.0-only */
> > + /*
> > + * xen/iommu-private.h
> > + *
> > + *
> > + * Copyright (C) 2023, Advanced Micro Devices, Inc. All Rights Reserved.
> > + * Written by Vikram Garhwal <vikram.garhwal@xxxxxxx>
> > + *
> > + */
> > +#ifndef __XEN_IOMMU_PRIVATE_H__
> > +#define __XEN_IOMMU_PRIVATE_H__
> > +
> > +#ifdef CONFIG_HAS_DEVICE_TREE
> > +#include <xen/device_tree.h>
> > +bool_t iommu_dt_device_is_assigned_locked(const struct dt_device_node 
> > *dev);
> > +#endif
> > +
> > +#endif /* __XEN_IOMMU_PRIVATE_H__ */
> > +/*
> > + * Local variables:
> > + * mode: C
> > + * c-file-style: "BSD"
> > + * c-basic-offset: 4
> > + * tab-width: 4
> > + * indent-tabs-mode: nil
> > + * End:
> > + */
> 
> -- 
> Julien Grall