[PATCH v6 5/6] iommu/dma: Allow a single FQ in addition to per-CPU FQs

Niklas Schnelle
2023-02-15T12:04:18Z

In some virtualized environments, including s390 paged memory guests,
IOTLB flushes are used to update IOMMU shadow tables. Due to this, they
are much more expensive than in typical bare metal environments or
non-paged s390 guests. In addition they may parallelize more poorly in
virtualized environments. This changes the trade off for flushing IOVAs
such that minimizing the number of IOTLB flushes trumps any benefit of
cheaper queuing operations or increased paralellism.

In this scenario per-CPU flush queues pose several problems. Firstly
per-CPU memory is often quite limited prohibiting larger queues.
Secondly collecting IOVAs per-CPU but flushing via a global timeout
reduces the number of IOVAs flushed for each timeout especially on s390
where PCI interrupts may not be bound to a specific CPU.

Let's introduce a single flush queue mode that reuses the same queue
logic but only allocates a single global queue. This mode can be
selected as a flag bit in a new dma_iommu_options struct which can be
modified from its defaults by IOMMU drivers implementing a new
ops.tune_dma_iommu() callback. As a first user the s390 IOMMU driver
selects the single queue mode if IOTLB flushes are needed on map which
indicates shadow table use. With the unchanged small FQ size and
timeouts this setting is worse than per-CPU queues but a follow up patch
will make the FQ size and timeout variable. Together this allows the
common IOVA flushing code to more closely resemble the global flush
behavior used on s390's previous internal DMA API implementation.

Link: https://lore.kernel.org/linux-iommu/3e402947-61f9-b7e8-1414-fde006257b6f@arm.com/
Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
---
v5 -> v6:
- Reworked so as not to introduce a new IOMMU domain type

drivers/iommu/dma-iommu.c | 161 ++++++++++++++++++++++++++++---------
drivers/iommu/dma-iommu.h | 4 +-
drivers/iommu/iommu.c | 4 +-
drivers/iommu/s390-iommu.c | 10 +++
include/linux/iommu.h | 21 +++++
5 files changed, 157 insertions(+), 43 deletions(-)

diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
index f798c44e0903..8f15a89ed7ab 100644
--- a/drivers/iommu/dma-iommu.c
+++ b/drivers/iommu/dma-iommu.c
@@ -48,8 +48,11 @@ struct iommu_dma_cookie {
/* Full allocator for IOMMU_DMA_IOVA_COOKIE */
struct {
struct iova_domain iovad;
-
- struct iova_fq __percpu *fq; /* Flush queue */
+ /* Flush queue */
+ union {
+ struct iova_fq *single_fq;
+ struct iova_fq __percpu *percpu_fq;
+ };
/* Number of TLB flushes that have been started */
atomic64_t fq_flush_start_cnt;
/* Number of TLB flushes that have been finished */
@@ -66,6 +69,8 @@ struct iommu_dma_cookie {

/* Domain for flush queue callback; NULL if flush queue not in use */
struct iommu_domain *fq_domain;
+ /* Options for dma-iommu use */
+ struct dma_iommu_options options;
struct mutex mutex;
};

@@ -151,25 +156,44 @@ static void fq_flush_iotlb(struct iommu_dma_cookie *cookie)
atomic64_inc(&cookie->fq_flush_finish_cnt);
}

-static void fq_flush_timeout(struct timer_list *t)
+static void fq_flush_percpu(struct iommu_dma_cookie *cookie)
{
- struct iommu_dma_cookie *cookie = from_timer(cookie, t, fq_timer);
int cpu;

- atomic_set(&cookie->fq_timer_on, 0);
- fq_flush_iotlb(cookie);
-
for_each_possible_cpu(cpu) {
unsigned long flags;
struct iova_fq *fq;

- fq = per_cpu_ptr(cookie->fq, cpu);
+ fq = per_cpu_ptr(cookie->percpu_fq, cpu);
spin_lock_irqsave(&fq->lock, flags);
fq_ring_free(cookie, fq);
spin_unlock_irqrestore(&fq->lock, flags);
}
}

+static void fq_flush_single(struct iommu_dma_cookie *cookie)
+{
+ struct iova_fq *fq = cookie->single_fq;
+ unsigned long flags;
+
+ spin_lock_irqsave(&fq->lock, flags);
+ fq_ring_free(cookie, fq);
+ spin_unlock_irqrestore(&fq->lock, flags);
+}
+
+static void fq_flush_timeout(struct timer_list *t)
+{
+ struct iommu_dma_cookie *cookie = from_timer(cookie, t, fq_timer);
+
+ atomic_set(&cookie->fq_timer_on, 0);
+ fq_flush_iotlb(cookie);
+
+ if (cookie->options.flags & IOMMU_DMA_OPTS_SINGLE_QUEUE)
+ fq_flush_single(cookie);
+ else
+ fq_flush_percpu(cookie);
+}
+
static void queue_iova(struct iommu_dma_cookie *cookie,
unsigned long pfn, unsigned long pages,
struct list_head *freelist)
@@ -187,7 +211,11 @@ static void queue_iova(struct iommu_dma_cookie *cookie,
*/
smp_mb();

- fq = raw_cpu_ptr(cookie->fq);
+ if (cookie->options.flags & IOMMU_DMA_OPTS_SINGLE_QUEUE)
+ fq = cookie->single_fq;
+ else
+ fq = raw_cpu_ptr(cookie->percpu_fq);
+
spin_lock_irqsave(&fq->lock, flags);

/*
@@ -218,58 +246,114 @@ static void queue_iova(struct iommu_dma_cookie *cookie,
jiffies + msecs_to_jiffies(IOVA_FQ_TIMEOUT));
}

-static void iommu_dma_free_fq(struct iommu_dma_cookie *cookie)
+static void iommu_dma_free_fq_single(struct iova_fq *fq)
{
- int cpu, idx;
+ int idx;

- if (!cookie->fq)
+ if (!fq)
return;
+ fq_ring_for_each(idx, fq)
+ put_pages_list(&fq->entries[idx].freelist);
+ vfree(fq);
+}
+
+static void iommu_dma_free_fq_percpu(struct iova_fq __percpu *percpu_fq)
+{
+ int cpu, idx;

- del_timer_sync(&cookie->fq_timer);
/* The IOVAs will be torn down separately, so just free our queued pages */
for_each_possible_cpu(cpu) {
- struct iova_fq *fq = per_cpu_ptr(cookie->fq, cpu);
+ struct iova_fq *fq = per_cpu_ptr(percpu_fq, cpu);

fq_ring_for_each(idx, fq)
put_pages_list(&fq->entries[idx].freelist);
}

- free_percpu(cookie->fq);
+ free_percpu(percpu_fq);
+}
+
+static void iommu_dma_free_fq(struct iommu_dma_cookie *cookie)
+{
+ if (!cookie->fq_domain)
+ return;
+
+ del_timer_sync(&cookie->fq_timer);
+ if (cookie->options.flags & IOMMU_DMA_OPTS_SINGLE_QUEUE)
+ iommu_dma_free_fq_single(cookie->single_fq);
+ else
+ iommu_dma_free_fq_percpu(cookie->percpu_fq);
+}
+
+
+static void iommu_dma_init_one_fq(struct iova_fq *fq)
+{
+ int i;
+
+ fq->head = 0;
+ fq->tail = 0;
+
+ spin_lock_init(&fq->lock);
+
+ for (i = 0; i < IOVA_FQ_SIZE; i++)
+ INIT_LIST_HEAD(&fq->entries[i].freelist);
+}
+
+static int iommu_dma_init_fq_single(struct iommu_dma_cookie *cookie)
+{
+ struct iova_fq *queue;
+
+ queue = vzalloc(sizeof(*queue));
+ if (!queue)
+ return -ENOMEM;
+ iommu_dma_init_one_fq(queue);
+ cookie->single_fq = queue;
+
+ return 0;
+}
+
+static int iommu_dma_init_fq_percpu(struct iommu_dma_cookie *cookie)
+{
+ struct iova_fq __percpu *queue;
+ int cpu;
+
+ queue = alloc_percpu(struct iova_fq);
+ if (!queue)
+ return -ENOMEM;
+
+ for_each_possible_cpu(cpu)
+ iommu_dma_init_one_fq(per_cpu_ptr(queue, cpu));
+ cookie->percpu_fq = queue;
+ return 0;
}

/* sysfs updates are serialised by the mutex of the group owning @domain */
-int iommu_dma_init_fq(struct iommu_domain *domain)
+int iommu_dma_init_fq(struct device *dev, struct iommu_domain *domain)
{
struct iommu_dma_cookie *cookie = domain->iova_cookie;
- struct iova_fq __percpu *queue;
- int i, cpu;
+ const struct iommu_ops *ops = dev_iommu_ops(dev);
+ int rc;

if (cookie->fq_domain)
return 0;

+ if (ops->tune_dma_iommu)
+ ops->tune_dma_iommu(dev, &cookie->options);
+
atomic64_set(&cookie->fq_flush_start_cnt, 0);
atomic64_set(&cookie->fq_flush_finish_cnt, 0);

- queue = alloc_percpu(struct iova_fq);
- if (!queue) {
- pr_warn("iova flush queue initialization failed\n");
- return -ENOMEM;
- }
-
- for_each_possible_cpu(cpu) {
- struct iova_fq *fq = per_cpu_ptr(queue, cpu);
-
- fq->head = 0;
- fq->tail = 0;
-
- spin_lock_init(&fq->lock);
+ if (cookie->options.flags & IOMMU_DMA_OPTS_SINGLE_QUEUE)
+ rc = iommu_dma_init_fq_single(cookie);
+ else
+ rc = iommu_dma_init_fq_percpu(cookie);

- for (i = 0; i < IOVA_FQ_SIZE; i++)
- INIT_LIST_HEAD(&fq->entries[i].freelist);
+ if (rc) {
+ pr_warn("iova flush queue initialization failed\n");
+ /* fall back to strict mode */
+ domain->type = IOMMU_DOMAIN_DMA;
+ return rc;
}

- cookie->fq = queue;
-
timer_setup(&cookie->fq_timer, fq_flush_timeout, 0);
atomic_set(&cookie->fq_timer_on, 0);
/*
@@ -296,6 +380,7 @@ static struct iommu_dma_cookie *cookie_alloc(enum iommu_dma_cookie_type type)
if (cookie) {
INIT_LIST_HEAD(&cookie->msi_page_list);
cookie->type = type;
+ cookie->options.flags = IOMMU_DMA_OPTS_PER_CPU_QUEUE;
}
return cookie;
}
@@ -582,9 +667,9 @@ static int iommu_dma_init_domain(struct iommu_domain *domain, dma_addr_t base,
if (ret)
goto done_unlock;

- /* If the FQ fails we can simply fall back to strict mode */
- if (domain->type == IOMMU_DOMAIN_DMA_FQ && iommu_dma_init_fq(domain))
- domain->type = IOMMU_DOMAIN_DMA;
+ /* If the FQ fails we fall back to strict mode */
+ if (domain->type == IOMMU_DOMAIN_DMA_FQ)
+ iommu_dma_init_fq(dev, domain);

ret = iova_reserve_iommu_regions(dev, domain);

diff --git a/drivers/iommu/dma-iommu.h b/drivers/iommu/dma-iommu.h
index 942790009292..4f727ab56d3c 100644
--- a/drivers/iommu/dma-iommu.h
+++ b/drivers/iommu/dma-iommu.h
@@ -12,7 +12,7 @@
int iommu_get_dma_cookie(struct iommu_domain *domain);
void iommu_put_dma_cookie(struct iommu_domain *domain);

-int iommu_dma_init_fq(struct iommu_domain *domain);
+int iommu_dma_init_fq(struct device *dev, struct iommu_domain *domain);

void iommu_dma_get_resv_regions(struct device *dev, struct list_head *list);

@@ -20,7 +20,7 @@ extern bool iommu_dma_forcedac;

#else /* CONFIG_IOMMU_DMA */

-static inline int iommu_dma_init_fq(struct iommu_domain *domain)
+static inline int iommu_dma_init_fq(struct device *dev, struct iommu_domain *domain)
{
return -EINVAL;
}
diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
index 5565e510f7d2..9c48dcdf0d2c 100644
--- a/drivers/iommu/iommu.c
+++ b/drivers/iommu/iommu.c
@@ -2897,9 +2897,7 @@ static int iommu_change_dev_def_domain(struct iommu_group *group,

/* We can bring up a flush queue without tearing down the domain */
if (type == IOMMU_DOMAIN_DMA_FQ && prev_dom->type == IOMMU_DOMAIN_DMA) {
- ret = iommu_dma_init_fq(prev_dom);
- if (!ret)
- prev_dom->type = IOMMU_DOMAIN_DMA_FQ;
+ ret = iommu_dma_init_fq(dev, prev_dom);
goto out;
}

diff --git a/drivers/iommu/s390-iommu.c b/drivers/iommu/s390-iommu.c
index 030f54dbee89..ea7232922fc5 100644
--- a/drivers/iommu/s390-iommu.c
+++ b/drivers/iommu/s390-iommu.c
@@ -458,6 +458,15 @@ static void s390_iommu_get_resv_regions(struct device *dev,
}
}

+static void s390_iommu_tune_dma_iommu(struct device *dev,
+ struct dma_iommu_options *options)
+{
+ struct zpci_dev *zdev = to_zpci_dev(dev);
+
+ if (zdev->tlb_refresh)
+ options->flags |= IOMMU_DMA_OPTS_SINGLE_QUEUE;
+}
+
static struct iommu_device *s390_iommu_probe_device(struct device *dev)
{
struct zpci_dev *zdev;
@@ -797,6 +806,7 @@ static const struct iommu_ops s390_iommu_ops = {
.device_group = generic_device_group,
.pgsize_bitmap = SZ_4K,
.get_resv_regions = s390_iommu_get_resv_regions,
+ .tune_dma_iommu = s390_iommu_tune_dma_iommu,
.default_domain_ops = &(const struct iommu_domain_ops) {
.attach_dev = s390_iommu_attach_device,
.detach_dev = s390_iommu_detach_device,
diff --git a/include/linux/iommu.h b/include/linux/iommu.h
index e7f76599f09e..071ac01e24ff 100644
--- a/include/linux/iommu.h
+++ b/include/linux/iommu.h
@@ -220,6 +220,21 @@ struct iommu_iotlb_gather {
bool queued;
};

+/**
+ * struct dma_iommu_options - Options for dma-iommu
+ *
+ * @flags: Flag bits for enabling/disabling dma-iommu settings
+ *
+ * This structure is intended to provide IOMMU drivers a way to influence the
+ * behavior of the dma-iommu DMA API implementation. This allows optimizing for
+ * example for a virtualized environment with slow IOTLB flushes.
+ */
+struct dma_iommu_options {
+#define IOMMU_DMA_OPTS_PER_CPU_QUEUE (0L << 0)
+#define IOMMU_DMA_OPTS_SINGLE_QUEUE (1L << 0)
+ u64 flags;
+};
+
/**
* struct iommu_ops - iommu ops and capabilities
* @capable: check capability
@@ -240,6 +255,9 @@ struct iommu_iotlb_gather {
* - IOMMU_DOMAIN_IDENTITY: must use an identity domain
* - IOMMU_DOMAIN_DMA: must use a dma domain
* - 0: use the default setting
+ * @tune_dma_iommu: Allows the IOMMU driver to modify the default
+ * options of the dma-iommu layer for a specific
+ * device.
* @default_domain_ops: the default ops for domains
* @remove_dev_pasid: Remove any translation configurations of a specific
* pasid, so that any DMA transactions with this pasid
@@ -275,6 +293,9 @@ struct iommu_ops {
int (*def_domain_type)(struct device *dev);
void (*remove_dev_pasid)(struct device *dev, ioasid_t pasid);

+ void (*tune_dma_iommu)(struct device *dev,
+ struct dma_iommu_options *options);
+
const struct iommu_domain_ops *default_domain_ops;
unsigned long pgsize_bitmap;
struct module *owner;
--
2.37.2