Date:   Wed, 8 Feb 2023 08:08:42 +0000
From:   "Liu, Yi L" <yi.l.liu@...el.com>
To:     Nicolin Chen <nicolinc@...dia.com>,
        "jgg@...dia.com" <jgg@...dia.com>,
        "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>
CC:     "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 08/10] iommufd/device: Use iommu_group_replace_domain()

Hi Nic,

> From: Nicolin Chen <nicolinc@...dia.com>
> Sent: Wednesday, February 8, 2023 5:18 AM
> 
> iommu_group_replace_domain() is introduced to support use cases where
> an
> iommu_group can be attached to a new domain without getting detached
> from
> the old one. This replacement feature will be useful, for cases such as:
> 1) vPASID mode, when a guest wants to replace a single pasid (PASID=0)
>    table with a larger table (PASID=N)
> 2) Nesting mode, when switching the attaching device from an S2 domain
>    to an S1 domain, or when switching between relevant S1 domains.
> as it allows these cases to switch seamlessly without a DMA disruption.
> 
> So, call iommu_group_replace_domain() in the
> iommufd_device_do_attach().
> And add a __iommmufd_device_detach helper to allow the replace routine
> to
> do a partial detach on the current hwpt that's being replaced. Though the
> updated locking logic is overcomplicated, it will be eased, once those
> iopt_table_add/remove_ioas and list_add/del calls are moved to hwpt's
> allocation/destroy() functions in the coming nesting series, as that'll
> depend on a new ->domain_alloc_user op in the iommu core.
> 
> Also, block replace operations that are from/to auto_domains, i.e. only
> user-allocated hw_pagetables can be replaced or replaced with.
> 
> Signed-off-by: Nicolin Chen <nicolinc@...dia.com>
> ---
>  drivers/iommu/iommufd/device.c          | 101 +++++++++++++++++-------
>  drivers/iommu/iommufd/iommufd_private.h |   2 +
>  2 files changed, 76 insertions(+), 27 deletions(-)
> 
> diff --git a/drivers/iommu/iommufd/device.c
> b/drivers/iommu/iommufd/device.c
> index b8c3e3baccb5..8a9834fc129a 100644
> --- a/drivers/iommu/iommufd/device.c
> +++ b/drivers/iommu/iommufd/device.c
> @@ -9,6 +9,8 @@
>  #include "io_pagetable.h"
>  #include "iommufd_private.h"
> 
> +MODULE_IMPORT_NS(IOMMUFD_INTERNAL);
> +
>  static bool allow_unsafe_interrupts;
>  module_param(allow_unsafe_interrupts, bool, S_IRUGO | S_IWUSR);
>  MODULE_PARM_DESC(
> @@ -194,9 +196,61 @@ static bool
> iommufd_hw_pagetable_has_group(struct iommufd_hw_pagetable *hwpt,
>  	return false;
>  }
> 
> +/**
> + * __iommmufd_device_detach - Detach a device from idev->hwpt to
> new_hwpt

This function doesn't do anything to make this device attached to new_hwpt.
It is done in the iommufd_device_attach_ioas(). New_hwpt here indicates if
this detach requires to do some extra thing. E.g. remove reserved iova from
the idev->hwpt->ioas. So may just say " Detach a device from idev->hwpt",
and explain the usage of new_hwpt in the below.

> + * @idev: device to detach
> + * @new_hwpt: new hw_pagetable to attach (pass in NULL for a simple
> detach)

The new hw_pagetable to be attached.

> + * @detach_group: flag to call iommu_detach_group
> + *
> + * This is a cleanup helper shared by the replace and detach routines.
> Comparing
> + * to a detach routine, a replace routine only needs a partial detach
> procedure:
> + * it does not need the iommu_detach_group(); it will attach the device to
> a new
> + * hw_pagetable after a partial detach from the currently attached
> hw_pagetable,
> + * so certain steps can be skipped if two hw_pagetables have the same
> IOAS.
> + */
> +static void __iommmufd_device_detach(struct iommufd_device *idev,
> +				     struct iommufd_hw_pagetable
> *new_hwpt,
> +				     bool detach_group)
> +{
> +	struct iommufd_hw_pagetable *hwpt = idev->hwpt;
> +	struct iommufd_ioas *new_ioas = NULL;
> +
> +	if (new_hwpt)
> +		new_ioas = new_hwpt->ioas;
> +
> +	mutex_lock(&hwpt->devices_lock);
> +	list_del(&idev->devices_item);
> +	if (hwpt->ioas != new_ioas)
> +		mutex_lock(&hwpt->ioas->mutex);

The lock order is mostly hwpt->ioas->mutex and then hwpt->devices_lock.
See the iommufd_device_auto_get_domain(). If possible, may switch the
order sequence here. Also, rename hwpt to be cur_hwpt, this may help
reviewers to distinguish it from the hwpt in the caller of this function. It
looks to be a deadlock at first look, but not after closer reading.

> +	if (!iommufd_hw_pagetable_has_group(hwpt, idev->group)) {
> +		if (list_empty(&hwpt->devices)) {
> +			iopt_table_remove_domain(&hwpt->ioas->iopt,
> +						 hwpt->domain);
> +			list_del(&hwpt->hwpt_item);
> +		}
> +		if (detach_group)
> +			iommu_detach_group(hwpt->domain, idev->group);
> +	}
> +	if (hwpt->ioas != new_ioas) {
> +		iopt_remove_reserved_iova(&hwpt->ioas->iopt, idev-
> >dev);
> +		mutex_unlock(&hwpt->ioas->mutex);
> +	}
> +	mutex_unlock(&hwpt->devices_lock);
> +
> +	if (hwpt->auto_domain)
> +		iommufd_object_destroy_user(idev->ictx, &hwpt->obj);
> +	else
> +		refcount_dec(&hwpt->obj.users);
> +
> +	idev->hwpt = NULL;
> +
> +	refcount_dec(&idev->obj.users);
> +}
> +
>  static int iommufd_device_do_attach(struct iommufd_device *idev,
>  				    struct iommufd_hw_pagetable *hwpt)
>  {
> +	struct iommufd_hw_pagetable *cur_hwpt = idev->hwpt;
>  	phys_addr_t sw_msi_start = PHYS_ADDR_MAX;
>  	int rc;
> 
> @@ -236,7 +290,7 @@ static int iommufd_device_do_attach(struct
> iommufd_device *idev,
>  	 * the group once for the first device that is in the group.
>  	 */
>  	if (!iommufd_hw_pagetable_has_group(hwpt, idev->group)) {
> -		rc = iommu_attach_group(hwpt->domain, idev->group);
> +		rc = iommu_group_replace_domain(idev->group, hwpt-
> >domain);
>  		if (rc)
>  			goto out_iova;
> 
> @@ -249,6 +303,10 @@ static int iommufd_device_do_attach(struct
> iommufd_device *idev,
>  		}
>  	}
> 
> +	/* Replace the cur_hwpt without iommu_detach_group() */
> +	if (cur_hwpt)
> +		__iommmufd_device_detach(idev, hwpt, false);
> +
>  	idev->hwpt = hwpt;
>  	refcount_inc(&hwpt->obj.users);
>  	list_add(&idev->devices_item, &hwpt->devices);
> @@ -256,7 +314,10 @@ static int iommufd_device_do_attach(struct
> iommufd_device *idev,
>  	return 0;
> 
>  out_detach:
> -	iommu_detach_group(hwpt->domain, idev->group);
> +	if (cur_hwpt)
> +		iommu_group_replace_domain(idev->group, cur_hwpt-
> >domain);
> +	else
> +		iommu_detach_group(hwpt->domain, idev->group);
>  out_iova:
>  	iopt_remove_reserved_iova(&hwpt->ioas->iopt, idev->dev);
>  out_unlock:
> @@ -345,6 +406,13 @@ int iommufd_device_attach(struct iommufd_device
> *idev, u32 *pt_id)
>  		struct iommufd_hw_pagetable *hwpt =
>  			container_of(pt_obj, struct
> iommufd_hw_pagetable, obj);
> 
> +		if (idev->hwpt == hwpt)
> +			goto out_done;
> +		if (idev->hwpt && idev->hwpt->auto_domain) {
> +			rc = -EBUSY;

This means if device was attached to an auto_created hwpt, then we
cannot replace it with a user allocated hwpt? If yes, this means the
replace is not available until user hwpt support, which is part of nesting.

> +			goto out_put_pt_obj;
> +		}
> +
>  		mutex_lock(&hwpt->ioas->mutex);
>  		rc = iommufd_device_do_attach(idev, hwpt);
>  		mutex_unlock(&hwpt->ioas->mutex);
> @@ -356,6 +424,8 @@ int iommufd_device_attach(struct iommufd_device
> *idev, u32 *pt_id)
>  		struct iommufd_ioas *ioas =
>  			container_of(pt_obj, struct iommufd_ioas, obj);
> 
> +		if (idev->hwpt)
> +			return -EBUSY;

So we don't allow ioas replacement for physical devices. Is it?
Looks like emulated devices allows it.

>  		rc = iommufd_device_auto_get_domain(idev, ioas);
>  		if (rc)
>  			goto out_put_pt_obj;
> @@ -367,6 +437,7 @@ int iommufd_device_attach(struct iommufd_device
> *idev, u32 *pt_id)
>  	}
> 
>  	refcount_inc(&idev->obj.users);
> +out_done:
>  	*pt_id = idev->hwpt->obj.id;
>  	rc = 0;
> 
> @@ -385,31 +456,7 @@
> EXPORT_SYMBOL_NS_GPL(iommufd_device_attach, IOMMUFD);
>   */
>  void iommufd_device_detach(struct iommufd_device *idev)
>  {
> -	struct iommufd_hw_pagetable *hwpt = idev->hwpt;
> -
> -	mutex_lock(&hwpt->ioas->mutex);
> -	mutex_lock(&hwpt->devices_lock);
> -	list_del(&idev->devices_item);
> -	if (!iommufd_hw_pagetable_has_group(hwpt, idev->group)) {
> -		if (list_empty(&hwpt->devices)) {
> -			iopt_table_remove_domain(&hwpt->ioas->iopt,
> -						 hwpt->domain);
> -			list_del(&hwpt->hwpt_item);
> -		}
> -		iommu_detach_group(hwpt->domain, idev->group);
> -	}
> -	iopt_remove_reserved_iova(&hwpt->ioas->iopt, idev->dev);
> -	mutex_unlock(&hwpt->devices_lock);
> -	mutex_unlock(&hwpt->ioas->mutex);
> -
> -	if (hwpt->auto_domain)
> -		iommufd_object_destroy_user(idev->ictx, &hwpt->obj);
> -	else
> -		refcount_dec(&hwpt->obj.users);
> -
> -	idev->hwpt = NULL;
> -
> -	refcount_dec(&idev->obj.users);
> +	__iommmufd_device_detach(idev, NULL, true);
>  }
>  EXPORT_SYMBOL_NS_GPL(iommufd_device_detach, IOMMUFD);
> 
> diff --git a/drivers/iommu/iommufd/iommufd_private.h
> b/drivers/iommu/iommufd/iommufd_private.h
> index 593138bb37b8..200c783800ad 100644
> --- a/drivers/iommu/iommufd/iommufd_private.h
> +++ b/drivers/iommu/iommufd/iommufd_private.h
> @@ -9,6 +9,8 @@
>  #include <linux/refcount.h>
>  #include <linux/uaccess.h>
> 
> +#include "../iommu-priv.h"
> +
>  struct iommu_domain;
>  struct iommu_group;
>  struct iommu_option;
> --
> 2.39.1

Regards,
Yi Liu

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