On Fri, 2023-08-18 at 19:16 +0100, Robin Murphy wrote:

On 2023-07-17 12:00, Niklas Schnelle wrote:

quoted

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/linux-iommu/3e402947-61f9-b7e8-1414-fde006257b6f@arm.com/ (local)
Reviewed-by: Matthew Rosato <mjrosato@linux.ibm.com> #s390
Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
---
  drivers/iommu/dma-iommu.c  | 178 +++++++++++++++++++++++++++++++++------------
  drivers/iommu/iommu.c      |  20 +----

The hunks in iommu.c appear to be an inadvertent reversion of patch #1?

A few bonus nits below which you can take or leave, but otherwise, with 
the rebase-mishap fixed:

Acked-by: Robin Murphy <robin.murphy@arm.com>

quoted

  drivers/iommu/s390-iommu.c |   3 +
  include/linux/iommu.h      |   2 +
  4 files changed, 142 insertions(+), 61 deletions(-)

diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
index e57724163835..4ada8b9749c9 100644
--- a/drivers/iommu/dma-iommu.c
+++ b/drivers/iommu/dma-iommu.c

@@ -43,14 +43,23 @@ enum iommu_dma_cookie_type {
  	IOMMU_DMA_MSI_COOKIE,
  };
  
+struct dma_iommu_options {
+#define IOMMU_DMA_OPTS_PER_CPU_QUEUE	0L

Nit: if the intent is to add more flags then that will no longer make 
sense, and if not then we may as well just have a bool ;)

quoted

+#define IOMMU_DMA_OPTS_SINGLE_QUEUE	BIT(0)

My thinking was that the above two options are mutually exclusive with
per-CPU encoded as BIT(0) unset and single queue as set. Then other
options could still use the other bits. It's true though that the below
use of IOMMU_DMA_OPTS_PER_CPU_QUEUE is a nop so maybe just drop that?
Or we could use an enum even if I don't forsee more than these 2 queue
types.

quoted

+	u64	flags;
+};
+
  

---8<---

quoted

  
+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);

@@ -152,23 +172,29 @@ static void fq_flush_iotlb(struct iommu_dma_cookie *cookie)
  	atomic64_inc(&cookie->fq_flush_finish_cnt);
  }
  
+static void fq_flush_percpu(struct iommu_dma_cookie *cookie)
+{
+	int cpu;
+
+	for_each_possible_cpu(cpu) {
+		struct iova_fq *fq;
+
+		fq = per_cpu_ptr(cookie->percpu_fq, cpu);
+		fq_ring_free(cookie, fq);
+	}
+}
+
  static void fq_flush_timeout(struct timer_list *t)
  {
  	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);
-		spin_lock_irqsave(&fq->lock, flags);
-		fq_ring_free(cookie, fq);
-		spin_unlock_irqrestore(&fq->lock, flags);
-	}
+	if (cookie->options.flags & IOMMU_DMA_OPTS_SINGLE_QUEUE)
+		fq_ring_free(cookie, cookie->single_fq);
+	else
+		fq_flush_percpu(cookie);

Nit: honestly I'd just inline that as:

	else for_each_possible_cpu(cpu)
		fq_ring_free(cookie, per_cpu_ptr(cookie->percpu_fq, cpu));

(possibly with extra braces if you don't feel brave enough to join the 
elite "else for" club...)

I might but it looks like checkpatch.pl isn't a fan:
...
ERROR: trailing statements should be on next line
#119: FILE: drivers/iommu/dma-iommu.c:185:
+       else for_each_possible_cpu(cpu)

So braces it is.

quoted

  }
  
  static void queue_iova(struct iommu_dma_cookie *cookie,

@@ -188,7 +214,11 @@ 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);
  
  	/*

@@ -196,11 +226,11 @@ 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 +249,90 @@ 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;
+
+	if (!fq)
+		return;

Nit: That should never be true if cookie->fq_domain was set

True and the _percpu variant doesn't check it either so will drop.

quoted

+	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.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));

Nit: vmalloc() - no need to zero the whole thing when the percpu path 
doesn't.

Done

quoted

+	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 +340,16 @@ 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.flags & 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);
  	/*

@@ -297,6 +376,7 @@ 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;

You see, this confused me into thinking it does something and I had to 
go back and check ;)

I have no real preference we can drop it, make it an enum, a bool or
keep it. I felt like this explicit assignment documented per-CPU as the
default so see my variant below but yes it does nothing since the
cookie was already kzalloced.

quoted

  	}
  	return cookie;
  }

@@ -614,10 +694,18 @@ 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 &&
-	    (!device_iommu_capable(dev, IOMMU_CAP_DEFERRED_FLUSH) || iommu_dma_init_fq(domain)))
-		domain->type = IOMMU_DOMAIN_DMA;
+	if (domain->type == IOMMU_DOMAIN_DMA_FQ) {
+		/* Expensive shadowing IOTLB flushes require some tuning */
+		if (dev->iommu->shadow_on_flush)
+			cookie->options.flags |= IOMMU_DMA_OPTS_SINGLE_QUEUE;

It would probably be better to set this regardless of the domain type, 
in case the FQ is only brought up later via sysfs.

quoted

+
+		/* If the FQ fails we can simply fall back to strict mode */
+		if (!device_iommu_capable(dev, IOMMU_CAP_DEFERRED_FLUSH) ||
+		    iommu_dma_init_fq(domain)) {
+			domain->type = IOMMU_DOMAIN_DMA;
+			cookie->options.flags &= ~IOMMU_DMA_OPTS_SINGLE_QUEUE;

...and either way there should be no need to clear it again - if it was 
true once it will always be true.

Cheers,
Robin.

I'm now experimenting with an iommu_dma_init_options() helper that gets
called from the main path of iommu_dma_init() here is how it looks like
at the end:

static void iommu_dma_init_options(struct iommu_dma_options *options, struct device *dev)
{
	/* Expensive shadowing IOTLB flushes do better with a single large queue */
	if (dev->iommu->shadow_on_flush) {
		options->flags = IOMMU_DMA_OPTS_SINGLE_QUEUE;
		options->fq_timeout = IOVA_SINGLE_FQ_TIMEOUT;
		options->fq_size = IOVA_SINGLE_FQ_SIZE;
	} else {
		options->flags = IOMMU_DMA_OPTS_PER_CPU_QUEUE;
		options->fq_size = IOVA_DEFAULT_FQ_SIZE;
		options->fq_timeout = IOVA_DEFAULT_FQ_TIMEOUT;
	}
}

Also I noticed that my options struct used the prefix "dma_iommu_"
while everything else is "iommu_dma_" so that got fixed too.

quoted

+		}
+	}
  
  	ret = iova_reserve_iommu_regions(dev, domain);
  

diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
index fd9f79731d6a..caaf563d38ae 100644
--- a/drivers/iommu/iommu.c
+++ b/drivers/iommu/iommu.c

@@ -2413,17 +2413,8 @@ int iommu_map(struct iommu_domain *domain, unsigned long iova,
  		return -EINVAL;
  
  	ret = __iommu_map(domain, iova, paddr, size, prot, gfp);
-	if (ret == 0 && ops->iotlb_sync_map) {
-		ret = ops->iotlb_sync_map(domain, iova, size);
-		if (ret)
-			goto out_err;
-	}
-
-	return ret;
-
-out_err:
-	/* undo mappings already done */
-	iommu_unmap(domain, iova, size);
+	if (ret == 0 && ops->iotlb_sync_map)
+		ops->iotlb_sync_map(domain, iova, size);
  
  	return ret;
  }

@@ -2564,11 +2555,8 @@ ssize_t iommu_map_sg(struct iommu_domain *domain, unsigned long iova,
  			sg = sg_next(sg);
  	}
  
-	if (ops->iotlb_sync_map) {
-		ret = ops->iotlb_sync_map(domain, iova, mapped);
-		if (ret)
-			goto out_err;
-	}
+	if (ops->iotlb_sync_map)
+		ops->iotlb_sync_map(domain, iova, mapped);
  	return mapped;

Good find that is indeed a rebase mishap ;-(

quoted

  
  out_err:

diff --git a/drivers/iommu/s390-iommu.c b/drivers/iommu/s390-iommu.c
index 020cc538e4c4..63c56a440c9d 100644
--- a/drivers/iommu/s390-iommu.c
+++ b/drivers/iommu/s390-iommu.c

@@ -468,6 +468,9 @@ 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 182cc4c71e62..c3687e066ed7 100644
--- a/include/linux/iommu.h
+++ b/include/linux/iommu.h

@@ -409,6 +409,7 @@ struct iommu_fault_param {
   * @priv:	 IOMMU Driver private data
   * @max_pasids:  number of PASIDs this device can consume
   * @attach_deferred: the dma domain attachment is deferred
+ * @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;

@@ -422,6 +423,7 @@ struct dev_iommu {
  	void				*priv;
  	u32				max_pasids;
  	u32				attach_deferred:1;
+	u32				shadow_on_flush:1;
  };
  
  int iommu_device_register(struct iommu_device *iommu,