Re: [PATCH 1/8] iommu: Add I/O ASID allocator

Jacob Pan
2019-06-11T12:23:20Z

On Mon, 10 Jun 2019 19:47:07 +0100
Jean-Philippe Brucker <jean-philippe.brucker@arm.com> wrote:

> Some devices might support multiple DMA address spaces, in particular
> those that have the PCI PASID feature. PASID (Process Address Space
> ID) allows to share process address spaces with devices (SVA),
> partition a device into VM-assignable entities (VFIO mdev) or simply
> provide multiple DMA address space to kernel drivers. Add a global
> PASID allocator usable by different drivers at the same time. Name it
> I/O ASID to avoid confusion with ASIDs allocated by arch code, which
> are usually a separate ID space.
>
> The IOASID space is global. Each device can have its own PASID space,
> but by convention the IOMMU ended up having a global PASID space, so
> that with SVA, each mm_struct is associated to a single PASID.
>
> The allocator is primarily used by IOMMU subsystem but in rare
> occasions drivers would like to allocate PASIDs for devices that
> aren't managed by an IOMMU, using the same ID space as IOMMU.
>
> Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
> Signed-off-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
> ---
> The most recent discussion on this patch was at:
> https://lkml.kernel.org/lkml/1556922737-76313-4-git-send-email-jacob.jun.pan@linux.intel.com/
> I fixed it up a bit following comments in that series, and removed the
> definitions for the custom allocator for now.
>
> There also is a new version that includes the custom allocator into
> this patch, but is currently missing the RCU fixes, at:
> https://lore.kernel.org/lkml/1560087862-57608-13-git-send-email-jacob.jun.pan@linux.intel.com/
> ---
> drivers/iommu/Kconfig | 4 ++
> drivers/iommu/Makefile | 1 +
> drivers/iommu/ioasid.c | 150
> +++++++++++++++++++++++++++++++++++++++++ include/linux/ioasid.h |
> 49 ++++++++++++++ 4 files changed, 204 insertions(+)
> create mode 100644 drivers/iommu/ioasid.c
> create mode 100644 include/linux/ioasid.h
>
> diff --git a/drivers/iommu/Kconfig b/drivers/iommu/Kconfig
> index 83664db5221d..9b45f70549a7 100644
> --- a/drivers/iommu/Kconfig
> +++ b/drivers/iommu/Kconfig
> @@ -3,6 +3,10 @@
> config IOMMU_IOVA
> tristate
>
> +# The IOASID library may also be used by non-IOMMU_API users
> +config IOASID
> + tristate
> +
> # IOMMU_API always gets selected by whoever wants it.
> config IOMMU_API
> bool
> diff --git a/drivers/iommu/Makefile b/drivers/iommu/Makefile
> index 8c71a15e986b..0efac6f1ec73 100644
> --- a/drivers/iommu/Makefile
> +++ b/drivers/iommu/Makefile
> @@ -7,6 +7,7 @@ obj-$(CONFIG_IOMMU_DMA) += dma-iommu.o
> obj-$(CONFIG_IOMMU_IO_PGTABLE) += io-pgtable.o
> obj-$(CONFIG_IOMMU_IO_PGTABLE_ARMV7S) += io-pgtable-arm-v7s.o
> obj-$(CONFIG_IOMMU_IO_PGTABLE_LPAE) += io-pgtable-arm.o
> +obj-$(CONFIG_IOASID) += ioasid.o
> obj-$(CONFIG_IOMMU_IOVA) += iova.o
> obj-$(CONFIG_OF_IOMMU) += of_iommu.o
> obj-$(CONFIG_MSM_IOMMU) += msm_iommu.o
> diff --git a/drivers/iommu/ioasid.c b/drivers/iommu/ioasid.c
> new file mode 100644
> index 000000000000..bbb771214fa9
> --- /dev/null
> +++ b/drivers/iommu/ioasid.c
> @@ -0,0 +1,150 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * I/O Address Space ID allocator. There is one global IOASID space,
> split into
> + * subsets. Users create a subset with DECLARE_IOASID_SET, then
> allocate and
> + * free IOASIDs with ioasid_alloc and ioasid_free.
> + */
> +#include <linux/ioasid.h>
> +#include <linux/module.h>
> +#include <linux/slab.h>
> +#include <linux/spinlock.h>
> +#include <linux/xarray.h>
> +
> +struct ioasid_data {
> + ioasid_t id;
> + struct ioasid_set *set;
> + void *private;
> + struct rcu_head rcu;
> +};
> +
> +static DEFINE_XARRAY_ALLOC(ioasid_xa);
> +
> +/**
> + * ioasid_set_data - Set private data for an allocated ioasid
> + * @ioasid: the ID to set data
> + * @data: the private data
> + *
> + * For IOASID that is already allocated, private data can be set
> + * via this API. Future lookup can be done via ioasid_find.
> + */
> +int ioasid_set_data(ioasid_t ioasid, void *data)
> +{
> + struct ioasid_data *ioasid_data;
> + int ret = 0;
> +
> + xa_lock(&ioasid_xa);
Just wondering if this is necessary, since xa_load is under
rcu_read_lock and we are not changing anything internal to xa. For
custom allocator I still need to have the mutex against allocator
removal.
> + ioasid_data = xa_load(&ioasid_xa, ioasid);
> + if (ioasid_data)
> + rcu_assign_pointer(ioasid_data->private, data);
it is good to publish and have barrier here. But I just wonder even for
weakly ordered machine, this pointer update is quite far away from its
data update.
> + else
> + ret = -ENOENT;
> + xa_unlock(&ioasid_xa);
> +
> + /*
> + * Wait for readers to stop accessing the old private data,
> so the
> + * caller can free it.
> + */
> + if (!ret)
> + synchronize_rcu();
> +
I will add that to my next version to check ret value.

Thanks,

Jacob
> + return ret;
> +}
> +EXPORT_SYMBOL_GPL(ioasid_set_data);
> +
> +/**
> + * ioasid_alloc - Allocate an IOASID
> + * @set: the IOASID set
> + * @min: the minimum ID (inclusive)
> + * @max: the maximum ID (inclusive)
> + * @private: data private to the caller
> + *
> + * Allocate an ID between @min and @max. The @private pointer is
> stored
> + * internally and can be retrieved with ioasid_find().
> + *
> + * Return: the allocated ID on success, or %INVALID_IOASID on
> failure.
> + */
> +ioasid_t ioasid_alloc(struct ioasid_set *set, ioasid_t min, ioasid_t
> max,
> + void *private)
> +{
> + u32 id = INVALID_IOASID;
> + struct ioasid_data *data;
> +
> + data = kzalloc(sizeof(*data), GFP_KERNEL);
> + if (!data)
> + return INVALID_IOASID;
> +
> + data->set = set;
> + data->private = private;
> +
> + if (xa_alloc(&ioasid_xa, &id, data, XA_LIMIT(min, max),
> GFP_KERNEL)) {
> + pr_err("Failed to alloc ioasid from %d to %d\n",
> min, max);
> + goto exit_free;
> + }
> + data->id = id;
> +
> +exit_free:
> + if (id == INVALID_IOASID) {
> + kfree(data);
> + return INVALID_IOASID;
> + }
> + return id;
> +}
> +EXPORT_SYMBOL_GPL(ioasid_alloc);
> +
> +/**
> + * ioasid_free - Free an IOASID
> + * @ioasid: the ID to remove
> + */
> +void ioasid_free(ioasid_t ioasid)
> +{
> + struct ioasid_data *ioasid_data;
> +
> + ioasid_data = xa_erase(&ioasid_xa, ioasid);
> +
> + kfree_rcu(ioasid_data, rcu);
> +}
> +EXPORT_SYMBOL_GPL(ioasid_free);
> +
> +/**
> + * ioasid_find - Find IOASID data
> + * @set: the IOASID set
> + * @ioasid: the IOASID to find
> + * @getter: function to call on the found object
> + *
> + * The optional getter function allows to take a reference to the
> found object
> + * under the rcu lock. The function can also check if the object is
> still valid:
> + * if @getter returns false, then the object is invalid and NULL is
> returned.
> + *
> + * If the IOASID has been allocated for this set, return the private
> pointer
> + * passed to ioasid_alloc. Private data can be NULL if not set.
> Return an error
> + * if the IOASID is not found or does not belong to the set.
> + */
> +void *ioasid_find(struct ioasid_set *set, ioasid_t ioasid,
> + bool (*getter)(void *))
> +{
> + void *priv = NULL;
> + struct ioasid_data *ioasid_data;
> +
> + rcu_read_lock();
> + ioasid_data = xa_load(&ioasid_xa, ioasid);
> + if (!ioasid_data) {
> + priv = ERR_PTR(-ENOENT);
> + goto unlock;
> + }
> + if (set && ioasid_data->set != set) {
> + /* data found but does not belong to the set */
> + priv = ERR_PTR(-EACCES);
> + goto unlock;
> + }
> + /* Now IOASID and its set is verified, we can return the
> private data */
> + priv = rcu_dereference(ioasid_data->private);
> + if (getter && !getter(priv))
> + priv = NULL;
> +unlock:
> + rcu_read_unlock();
> +
> + return priv;
> +}
> +EXPORT_SYMBOL_GPL(ioasid_find);
> +
> +MODULE_LICENSE("GPL");
> diff --git a/include/linux/ioasid.h b/include/linux/ioasid.h
> new file mode 100644
> index 000000000000..940212422b8f
> --- /dev/null
> +++ b/include/linux/ioasid.h
> @@ -0,0 +1,49 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef __LINUX_IOASID_H
> +#define __LINUX_IOASID_H
> +
> +#include <linux/types.h>
> +
> +#define INVALID_IOASID ((ioasid_t)-1)
> +typedef unsigned int ioasid_t;
> +
> +struct ioasid_set {
> + int dummy;
> +};
> +
> +#define DECLARE_IOASID_SET(name) struct ioasid_set name = { 0 }
> +
> +#if IS_ENABLED(CONFIG_IOASID)
> +ioasid_t ioasid_alloc(struct ioasid_set *set, ioasid_t min, ioasid_t
> max,
> + void *private);
> +void ioasid_free(ioasid_t ioasid);
> +
> +void *ioasid_find(struct ioasid_set *set, ioasid_t ioasid,
> + bool (*getter)(void *));
> +
> +int ioasid_set_data(ioasid_t ioasid, void *data);
> +
> +#else /* !CONFIG_IOASID */
> +static inline ioasid_t ioasid_alloc(struct ioasid_set *set, ioasid_t
> min,
> + ioasid_t max, void *private)
> +{
> + return INVALID_IOASID;
> +}
> +
> +static inline void ioasid_free(ioasid_t ioasid)
> +{
> +}
> +
> +static inline void *ioasid_find(struct ioasid_set *set, ioasid_t
> ioasid,
> + bool (*getter)(void *))
> +{
> + return NULL;
> +}
> +
> +static inline int ioasid_set_data(ioasid_t ioasid, void *data)
> +{
> + return -ENODEV;
> +}
> +
> +#endif /* CONFIG_IOASID */
> +#endif /* __LINUX_IOASID_H */

[Jacob Pan]