[v11,6/6] iommu/dma: Use a large flush queue and timeout for shadow_on_flush

Flush queues currently use a fixed compile time size of 256 entries.
This being a power of 2 allows the compiler to use shift and mask
instead of more expensive modulo operations. With per-CPU flush queues
larger queue sizes would hit per-CPU allocation limits, with a single
flush queue these limits do not apply however. Also with single queues
being particularly suitable for virtualized environments with expensive
IOTLB flushes these benefit especially from larger queues and thus fewer
flushes.

To this end re-order struct iova_fq so we can use a dynamic array and
introduce the flush queue size and timeouts as new options in the
dma_iommu_options struct. So as not to lose the shift and mask
optimization, use a power of 2 for the length and use explicit shift and
mask instead of letting the compiler optimize this.

A large queue size and 1 second timeout is then set for the shadow on
flush case set by s390 paged memory guests. This then brings performance
on par with the previous s390 specific DMA API implementation.

Reviewed-by: Matthew Rosato <mjrosato@linux.ibm.com> #s390
Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
---
 drivers/iommu/dma-iommu.c | 50 +++++++++++++++++++++++++++++++----------------
 1 file changed, 33 insertions(+), 17 deletions(-)

On 2023-07-17 12:00, Niklas Schnelle wrote:
> Flush queues currently use a fixed compile time size of 256 entries.
> This being a power of 2 allows the compiler to use shift and mask
> instead of more expensive modulo operations. With per-CPU flush queues
> larger queue sizes would hit per-CPU allocation limits, with a single
> flush queue these limits do not apply however. Also with single queues
> being particularly suitable for virtualized environments with expensive
> IOTLB flushes these benefit especially from larger queues and thus fewer
> flushes.
> 
> To this end re-order struct iova_fq so we can use a dynamic array and
> introduce the flush queue size and timeouts as new options in the
> dma_iommu_options struct. So as not to lose the shift and mask
> optimization, use a power of 2 for the length and use explicit shift and
> mask instead of letting the compiler optimize this.
> 
> A large queue size and 1 second timeout is then set for the shadow on
> flush case set by s390 paged memory guests. This then brings performance
> on par with the previous s390 specific DMA API implementation.
> 
> Reviewed-by: Matthew Rosato <mjrosato@linux.ibm.com> #s390
> Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
> ---
>   drivers/iommu/dma-iommu.c | 50 +++++++++++++++++++++++++++++++----------------
>   1 file changed, 33 insertions(+), 17 deletions(-)
> 
> diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
> index 4ada8b9749c9..bebc0804ff53 100644
> --- a/drivers/iommu/dma-iommu.c
> +++ b/drivers/iommu/dma-iommu.c
> @@ -46,7 +46,9 @@ enum iommu_dma_cookie_type {
>   struct dma_iommu_options {
>   #define IOMMU_DMA_OPTS_PER_CPU_QUEUE	0L
>   #define IOMMU_DMA_OPTS_SINGLE_QUEUE	BIT(0)
> -	u64	flags;
> +	u64		flags;
> +	size_t		fq_size;
> +	unsigned int	fq_timeout;
>   };
>   
>   struct iommu_dma_cookie {
> @@ -95,10 +97,12 @@ static int __init iommu_dma_forcedac_setup(char *str)
>   early_param("iommu.forcedac", iommu_dma_forcedac_setup);
>   
>   /* Number of entries per flush queue */
> -#define IOVA_FQ_SIZE	256
> +#define IOVA_DEFAULT_FQ_SIZE	256
> +#define IOVA_SINGLE_FQ_SIZE	32768
>   
>   /* Timeout (in ms) after which entries are flushed from the queue */
> -#define IOVA_FQ_TIMEOUT	10
> +#define IOVA_DEFAULT_FQ_TIMEOUT	10
> +#define IOVA_SINGLE_FQ_TIMEOUT	1000
>   
>   /* Flush queue entry for deferred flushing */
>   struct iova_fq_entry {
> @@ -110,18 +114,19 @@ struct iova_fq_entry {
>   
>   /* Per-CPU flush queue structure */
>   struct iova_fq {
> -	struct iova_fq_entry entries[IOVA_FQ_SIZE];
> -	unsigned int head, tail;
>   	spinlock_t lock;
> +	unsigned int head, tail;
> +	unsigned int mod_mask;
> +	struct iova_fq_entry entries[];
>   };
>   
>   #define fq_ring_for_each(i, fq) \
> -	for ((i) = (fq)->head; (i) != (fq)->tail; (i) = ((i) + 1) % IOVA_FQ_SIZE)
> +	for ((i) = (fq)->head; (i) != (fq)->tail; (i) = ((i) + 1) & (fq)->mod_mask)
>   
>   static inline bool fq_full(struct iova_fq *fq)
>   {
>   	assert_spin_locked(&fq->lock);
> -	return (((fq->tail + 1) % IOVA_FQ_SIZE) == fq->head);
> +	return (((fq->tail + 1) & fq->mod_mask) == fq->head);
>   }
>   
>   static inline unsigned int fq_ring_add(struct iova_fq *fq)
> @@ -130,7 +135,7 @@ static inline unsigned int fq_ring_add(struct iova_fq *fq)
>   
>   	assert_spin_locked(&fq->lock);
>   
> -	fq->tail = (idx + 1) % IOVA_FQ_SIZE;
> +	fq->tail = (idx + 1) & fq->mod_mask;
>   
>   	return idx;
>   }
> @@ -152,7 +157,7 @@ static void fq_ring_free_locked(struct iommu_dma_cookie *cookie, struct iova_fq
>   			       fq->entries[idx].iova_pfn,
>   			       fq->entries[idx].pages);
>   
> -		fq->head = (fq->head + 1) % IOVA_FQ_SIZE;
> +		fq->head = (fq->head + 1) & fq->mod_mask;
>   	}
>   }
>   
> @@ -246,7 +251,7 @@ static void queue_iova(struct iommu_dma_cookie *cookie,
>   	if (!atomic_read(&cookie->fq_timer_on) &&
>   	    !atomic_xchg(&cookie->fq_timer_on, 1))
>   		mod_timer(&cookie->fq_timer,
> -			  jiffies + msecs_to_jiffies(IOVA_FQ_TIMEOUT));
> +			  jiffies + msecs_to_jiffies(cookie->options.fq_timeout));
>   }
>   
>   static void iommu_dma_free_fq_single(struct iova_fq *fq)
> @@ -287,27 +292,29 @@ static void iommu_dma_free_fq(struct iommu_dma_cookie *cookie)
>   		iommu_dma_free_fq_percpu(cookie->percpu_fq);
>   }
>   
> -static void iommu_dma_init_one_fq(struct iova_fq *fq)
> +static void iommu_dma_init_one_fq(struct iova_fq *fq, size_t fq_size)
>   {
>   	int i;
>   
>   	fq->head = 0;
>   	fq->tail = 0;
> +	fq->mod_mask = fq_size - 1;
>   
>   	spin_lock_init(&fq->lock);
>   
> -	for (i = 0; i < IOVA_FQ_SIZE; i++)
> +	for (i = 0; i < fq_size; i++)
>   		INIT_LIST_HEAD(&fq->entries[i].freelist);
>   }
>   
>   static int iommu_dma_init_fq_single(struct iommu_dma_cookie *cookie)
>   {
> +	size_t fq_size = cookie->options.fq_size;
>   	struct iova_fq *queue;
>   
> -	queue = vzalloc(sizeof(*queue));
> +	queue = vzalloc(struct_size(queue, entries, fq_size));
>   	if (!queue)
>   		return -ENOMEM;
> -	iommu_dma_init_one_fq(queue);
> +	iommu_dma_init_one_fq(queue, fq_size);
>   	cookie->single_fq = queue;
>   
>   	return 0;
> @@ -315,15 +322,17 @@ static int iommu_dma_init_fq_single(struct iommu_dma_cookie *cookie)
>   
>   static int iommu_dma_init_fq_percpu(struct iommu_dma_cookie *cookie)
>   {
> +	size_t fq_size = cookie->options.fq_size;
>   	struct iova_fq __percpu *queue;
>   	int cpu;
>   
> -	queue = alloc_percpu(struct iova_fq);
> +	queue = __alloc_percpu(struct_size(queue, entries, fq_size),
> +			       __alignof__(*queue));
>   	if (!queue)
>   		return -ENOMEM;
>   
>   	for_each_possible_cpu(cpu)
> -		iommu_dma_init_one_fq(per_cpu_ptr(queue, cpu));
> +		iommu_dma_init_one_fq(per_cpu_ptr(queue, cpu), fq_size);
>   	cookie->percpu_fq = queue;
>   	return 0;
>   }
> @@ -377,6 +386,8 @@ static struct iommu_dma_cookie *cookie_alloc(enum iommu_dma_cookie_type type)
>   		INIT_LIST_HEAD(&cookie->msi_page_list);
>   		cookie->type = type;
>   		cookie->options.flags = IOMMU_DMA_OPTS_PER_CPU_QUEUE;
> +		cookie->options.fq_size = IOVA_DEFAULT_FQ_SIZE;
> +		cookie->options.fq_timeout = IOVA_DEFAULT_FQ_TIMEOUT;

Similar comment as on the previous patch - it would be clearer and more 
robust to set fq_size and fq_timeout all in one place in the main path 
of iommu_dma_init_domain(). Otherwise,

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

>   	}
>   	return cookie;
>   }
> @@ -696,14 +707,19 @@ static int iommu_dma_init_domain(struct iommu_domain *domain, dma_addr_t base,
>   
>   	if (domain->type == IOMMU_DOMAIN_DMA_FQ) {
>   		/* Expensive shadowing IOTLB flushes require some tuning */
> -		if (dev->iommu->shadow_on_flush)
> +		if (dev->iommu->shadow_on_flush) {
>   			cookie->options.flags |= IOMMU_DMA_OPTS_SINGLE_QUEUE;
> +			cookie->options.fq_timeout = IOVA_SINGLE_FQ_TIMEOUT;
> +			cookie->options.fq_size = IOVA_SINGLE_FQ_SIZE;
> +		}
>   
>   		/* 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;
> +			cookie->options.fq_timeout = IOVA_DEFAULT_FQ_TIMEOUT;
> +			cookie->options.fq_size = IOVA_DEFAULT_FQ_SIZE;
>   		}
>   	}
>   
>