[PATCH v13 5/6] iommu/dma: Allow a single FQ in addition to per-CPU FQs

Niklas Schnelle
2023-09-28T14:33:55Z

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 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 is selected by
dma-iommu if a newly introduced .shadow_on_flush flag is set in struct
dev_iommu. As a first user the s390 IOMMU driver sets this flag during
probe_device. 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/all/9a466109-01c5-96b0-bf03-304123f435ee@arm.com/
Acked-by: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: Matthew Rosato <mjrosato@linux.ibm.com> #s390
Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
---
drivers/iommu/dma-iommu.c | 168 ++++++++++++++++++++++++++++++++++-----------
drivers/iommu/s390-iommu.c | 3 +
include/linux/iommu.h | 2 +
3 files changed, 134 insertions(+), 39 deletions(-)

diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
index 4b1a88f514c9..a85ff75ad531 100644
--- a/drivers/iommu/dma-iommu.c
+++ b/drivers/iommu/dma-iommu.c
@@ -43,14 +43,26 @@ enum iommu_dma_cookie_type {
IOMMU_DMA_MSI_COOKIE,
};

+enum iommu_dma_queue_type {
+ IOMMU_DMA_OPTS_PER_CPU_QUEUE,
+ IOMMU_DMA_OPTS_SINGLE_QUEUE,
+};
+
+struct iommu_dma_options {
+ enum iommu_dma_queue_type qt;
+};
+
struct iommu_dma_cookie {
enum iommu_dma_cookie_type type;
union {
/* 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 */
@@ -67,6 +79,8 @@ 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 iommu_dma_options options;
struct mutex mutex;
};

@@ -124,7 +138,7 @@ static inline unsigned int fq_ring_add(struct iova_fq *fq)
return idx;
}

-static void fq_ring_free(struct iommu_dma_cookie *cookie, struct iova_fq *fq)
+static void fq_ring_free_locked(struct iommu_dma_cookie *cookie, struct iova_fq *fq)
{
u64 counter = atomic64_read(&cookie->fq_flush_finish_cnt);
unsigned int idx;
@@ -145,6 +159,15 @@ static void fq_ring_free(struct iommu_dma_cookie *cookie, struct iova_fq *fq)
}
}

+static void fq_ring_free(struct iommu_dma_cookie *cookie, struct iova_fq *fq)
+{
+ unsigned long flags;
+
+ spin_lock_irqsave(&fq->lock, flags);
+ fq_ring_free_locked(cookie, fq);
+ spin_unlock_irqrestore(&fq->lock, flags);
+}
+
static void fq_flush_iotlb(struct iommu_dma_cookie *cookie)
{
atomic64_inc(&cookie->fq_flush_start_cnt);
@@ -160,14 +183,11 @@ static void fq_flush_timeout(struct timer_list *t)
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);
- spin_lock_irqsave(&fq->lock, flags);
- fq_ring_free(cookie, fq);
- spin_unlock_irqrestore(&fq->lock, flags);
+ if (cookie->options.qt == IOMMU_DMA_OPTS_SINGLE_QUEUE) {
+ fq_ring_free(cookie, cookie->single_fq);
+ } else {
+ for_each_possible_cpu(cpu)
+ fq_ring_free(cookie, per_cpu_ptr(cookie->percpu_fq, cpu));
}
}

@@ -188,7 +208,11 @@ static void queue_iova(struct iommu_dma_cookie *cookie,
*/
smp_mb();

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

/*
@@ -196,11 +220,11 @@ static void queue_iova(struct iommu_dma_cookie *cookie,
* flushed out on another CPU. This makes the fq_full() check below less
* likely to be true.
*/
- fq_ring_free(cookie, fq);
+ fq_ring_free_locked(cookie, fq);

if (fq_full(fq)) {
fq_flush_iotlb(cookie);
- fq_ring_free(cookie, fq);
+ fq_ring_free_locked(cookie, fq);
}

idx = fq_ring_add(fq);
@@ -219,31 +243,88 @@ 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 idx;
+
+ 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;

- if (!cookie->fq)
- return;
-
- 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.qt == 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 = vmalloc(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)
{
struct iommu_dma_cookie *cookie = domain->iova_cookie;
- struct iova_fq __percpu *queue;
- int i, cpu;
+ int rc;

if (cookie->fq_domain)
return 0;
@@ -251,26 +332,16 @@ int iommu_dma_init_fq(struct iommu_domain *domain)
atomic64_set(&cookie->fq_flush_start_cnt, 0);
atomic64_set(&cookie->fq_flush_finish_cnt, 0);

- queue = alloc_percpu(struct iova_fq);
- if (!queue) {
+ if (cookie->options.qt == IOMMU_DMA_OPTS_SINGLE_QUEUE)
+ rc = iommu_dma_init_fq_single(cookie);
+ else
+ rc = iommu_dma_init_fq_percpu(cookie);
+
+ if (rc) {
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);
-
- for (i = 0; i < IOVA_FQ_SIZE; i++)
- INIT_LIST_HEAD(&fq->entries[i].freelist);
- }
-
- cookie->fq = queue;
-
timer_setup(&cookie->fq_timer, fq_flush_timeout, 0);
atomic_set(&cookie->fq_timer_on, 0);
/*
@@ -554,6 +625,23 @@ static bool dev_use_sg_swiotlb(struct device *dev, struct scatterlist *sg,
return false;
}

+/**
+ * iommu_dma_init_options - Initialize dma-iommu options
+ * @options: The options to be initialized
+ * @dev: Device the options are set for
+ *
+ * This allows tuning dma-iommu specific to device properties
+ */
+static void iommu_dma_init_options(struct iommu_dma_options *options,
+ struct device *dev)
+{
+ /* Shadowing IOTLB flushes do better with a single queue */
+ if (dev->iommu->shadow_on_flush)
+ options->qt = IOMMU_DMA_OPTS_SINGLE_QUEUE;
+ else
+ options->qt = IOMMU_DMA_OPTS_PER_CPU_QUEUE;
+}
+
/**
* iommu_dma_init_domain - Initialise a DMA mapping domain
* @domain: IOMMU domain previously prepared by iommu_get_dma_cookie()
@@ -614,6 +702,8 @@ static int iommu_dma_init_domain(struct iommu_domain *domain, dma_addr_t base,
if (ret)
goto done_unlock;

+ iommu_dma_init_options(&cookie->options, dev);
+
/* If the FQ fails we can simply fall back to strict mode */
if (domain->type == IOMMU_DOMAIN_DMA_FQ &&
(!device_iommu_capable(dev, IOMMU_CAP_DEFERRED_FLUSH) || iommu_dma_init_fq(domain)))
diff --git a/drivers/iommu/s390-iommu.c b/drivers/iommu/s390-iommu.c
index 5b5ba1cd5f00..9a5196f523de 100644
--- a/drivers/iommu/s390-iommu.c
+++ b/drivers/iommu/s390-iommu.c
@@ -463,6 +463,9 @@ static struct iommu_device *s390_iommu_probe_device(struct device *dev)
if (zdev->end_dma > ZPCI_TABLE_SIZE_RT - 1)
zdev->end_dma = ZPCI_TABLE_SIZE_RT - 1;

+ if (zdev->tlb_refresh)
+ dev->iommu->shadow_on_flush = 1;
+
return &zdev->iommu_dev;
}

diff --git a/include/linux/iommu.h b/include/linux/iommu.h
index 1eb638752781..0c4d8ae985ac 100644
--- a/include/linux/iommu.h
+++ b/include/linux/iommu.h
@@ -424,6 +424,7 @@ struct iommu_fault_param {
* @attach_deferred: the dma domain attachment is deferred
* @pci_32bit_workaround: Limit DMA allocations to 32-bit IOVAs
* @require_direct: device requires IOMMU_RESV_DIRECT regions
+ * @shadow_on_flush: IOTLB flushes are used to sync shadow tables
*
* TODO: migrate other per device data pointers under iommu_dev_data, e.g.
* struct iommu_group *iommu_group;
@@ -439,6 +440,7 @@ struct dev_iommu {
u32 attach_deferred:1;
u32 pci_32bit_workaround:1;
u32 require_direct:1;
+ u32 shadow_on_flush:1;
};

int iommu_device_register(struct iommu_device *iommu,

--
2.39.2