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Date: Wed, 15 Apr 2020 09:30:13 +0000
From: "Tian, Kevin" <kevin.tian@...el.com>
To: Lu Baolu <baolu.lu@...ux.intel.com>, Joerg Roedel <joro@...tes.org>
CC: "Raj, Ashok" <ashok.raj@...el.com>,
"jacob.jun.pan@...ux.intel.com" <jacob.jun.pan@...ux.intel.com>,
"Liu, Yi L" <yi.l.liu@...el.com>,
"iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v2 5/7] iommu/vt-d: Save prq descriptors in an internal
list
> From: Lu Baolu <baolu.lu@...ux.intel.com>
> Sent: Wednesday, April 15, 2020 1:26 PM
>
> Currently, the page request interrupt thread handles the page
> requests in the queue in this way:
>
> - Clear PPR bit to ensure new interrupt could come in;
> - Read and record the head and tail registers;
> - Handle all descriptors between head and tail;
> - Write tail to head register.
>
> This might cause some descriptors to be handles multiple times.
> An example sequence:
>
> - Thread A got scheduled with PRQ_1 and PRQ_2 in the queue;
> - Thread A clear the PPR bit and record the head and tail;
> - A new PRQ_3 comes and Thread B gets scheduled;
> - Thread B record the head and tail which includes PRQ_1
> and PRQ_2.
I may overlook something but isn't the prq interrupt thread
per iommu then why would two prq threads contend here?
Thanks,
Kevin
>
> As the result, PRQ_1 and PRQ_2 are handled twice in Thread_A and
> Thread_B.
>
> Thread_A Thread_B
> .--------. .--------.
> | | | |
> .--------. .--------.
> head| PRQ_1 | head| PRQ_1 |
> .--------. .--------.
> | PRQ_2 | | PRQ_2 |
> .--------. .--------.
> tail| | | PRQ_3 |
> .--------. .--------.
> | | tail| |
> '--------' '--------'
>
> To avoid this, probably, we need to apply a spinlock to ensure
> that PRQs are handled in a serialized way. But that means the
> intel_svm_process_prq() will be called with a spinlock held.
> This causes extra complexities in intel_svm_process_prq().
>
> This aims to make PRQ descriptors to be handled in a serialized
> way while remove the requirement of holding the spin lock in
> intel_svm_process_prq() by saving the descriptors in a list.
>
> Signed-off-by: Lu Baolu <baolu.lu@...ux.intel.com>
> ---
> drivers/iommu/intel-svm.c | 58 ++++++++++++++++++++++++++++++-------
> include/linux/intel-iommu.h | 2 ++
> 2 files changed, 49 insertions(+), 11 deletions(-)
>
> diff --git a/drivers/iommu/intel-svm.c b/drivers/iommu/intel-svm.c
> index a1921b462783..05aeb8ea51c4 100644
> --- a/drivers/iommu/intel-svm.c
> +++ b/drivers/iommu/intel-svm.c
> @@ -50,6 +50,8 @@ int intel_svm_enable_prq(struct intel_iommu *iommu)
> return ret;
> }
> iommu->pr_irq = irq;
> + INIT_LIST_HEAD(&iommu->prq_list);
> + spin_lock_init(&iommu->prq_lock);
>
> snprintf(iommu->prq_name, sizeof(iommu->prq_name), "dmar%d-
> prq", iommu->seq_id);
>
> @@ -698,6 +700,14 @@ struct page_req_dsc {
>
> #define PRQ_RING_MASK ((0x1000 << PRQ_ORDER) - 0x20)
>
> +struct page_req {
> + struct list_head list;
> + struct page_req_dsc desc;
> + unsigned int processing:1;
> + unsigned int drained:1;
> + unsigned int completed:1;
> +};
> +
> static bool access_error(struct vm_area_struct *vma, struct page_req_dsc
> *req)
> {
> unsigned long requested = 0;
> @@ -842,34 +852,60 @@ static void process_single_prq(struct intel_iommu
> *iommu,
> }
> }
>
> -static void intel_svm_process_prq(struct intel_iommu *iommu,
> - struct page_req_dsc *prq,
> - int head, int tail)
> +static void intel_svm_process_prq(struct intel_iommu *iommu)
> {
> - struct page_req_dsc *req;
> -
> - while (head != tail) {
> - req = &iommu->prq[head / sizeof(*req)];
> - process_single_prq(iommu, req);
> - head = (head + sizeof(*req)) & PRQ_RING_MASK;
> + struct page_req *req;
> + unsigned long flags;
> +
> + spin_lock_irqsave(&iommu->prq_lock, flags);
> + while (!list_empty(&iommu->prq_list)) {
> + req = list_first_entry(&iommu->prq_list, struct page_req, list);
> + if (!req->processing) {
> + req->processing = true;
> + spin_unlock_irqrestore(&iommu->prq_lock, flags);
> + process_single_prq(iommu, &req->desc);
> + spin_lock_irqsave(&iommu->prq_lock, flags);
> + req->completed = true;
> + } else if (req->completed) {
> + list_del(&req->list);
> + kfree(req);
> + } else {
> + break;
> + }
> }
> + spin_unlock_irqrestore(&iommu->prq_lock, flags);
> }
>
> static irqreturn_t prq_event_thread(int irq, void *d)
> {
> struct intel_iommu *iommu = d;
> + unsigned long flags;
> int head, tail;
>
> + spin_lock_irqsave(&iommu->prq_lock, flags);
> /*
> * Clear PPR bit before reading head/tail registers, to
> * ensure that we get a new interrupt if needed.
> */
> writel(DMA_PRS_PPR, iommu->reg + DMAR_PRS_REG);
> -
> tail = dmar_readq(iommu->reg + DMAR_PQT_REG) &
> PRQ_RING_MASK;
> head = dmar_readq(iommu->reg + DMAR_PQH_REG) &
> PRQ_RING_MASK;
> - intel_svm_process_prq(iommu, iommu->prq, head, tail);
> + while (head != tail) {
> + struct page_req_dsc *dsc;
> + struct page_req *req;
> +
> + dsc = &iommu->prq[head / sizeof(*dsc)];
> + req = kzalloc(sizeof (*req), GFP_ATOMIC);
> + if (!req)
> + break;
> + req->desc = *dsc;
> + list_add_tail(&req->list, &iommu->prq_list);
> + head = (head + sizeof(*dsc)) & PRQ_RING_MASK;
> + }
> dmar_writeq(iommu->reg + DMAR_PQH_REG, tail);
> + spin_unlock_irqrestore(&iommu->prq_lock, flags);
> +
> + intel_svm_process_prq(iommu);
>
> return IRQ_RETVAL(1);
> }
> diff --git a/include/linux/intel-iommu.h b/include/linux/intel-iommu.h
> index cca1e5f9aeaa..80715a59491c 100644
> --- a/include/linux/intel-iommu.h
> +++ b/include/linux/intel-iommu.h
> @@ -590,6 +590,8 @@ struct intel_iommu {
> #ifdef CONFIG_INTEL_IOMMU_SVM
> struct page_req_dsc *prq;
> unsigned char prq_name[16]; /* Name for PRQ interrupt */
> + struct list_head prq_list; /* pending page request list */
> + spinlock_t prq_lock; /* protect above pending list */
> struct ioasid_allocator_ops pasid_allocator; /* Custom allocator for
> PASIDs */
> #endif
> struct q_inval *qi; /* Queued invalidation info */
> --
> 2.17.1
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