Re: [PATCH v2 4/4] iommu/arm-smmu-v3: Poll for CMD_SYNC outside cmdq lock

[Will Deacon <will.deacon-5wv7dgnIgG8@public.gmane.org>]

Hi Robin,

Some of my comments on patch 3 are addressed here, but I'm really struggling
to convince myself that this algorithm is correct. My preference would
be to leave the code as it is for SMMUs that don't implement MSIs, but
comments below anyway because it's an interesting idea.

On Thu, Aug 31, 2017 at 02:44:28PM +0100, Robin Murphy wrote:
> Even without the MSI trick, we can still do a lot better than hogging
> the entire queue while it drains. All we actually need to do for the
> necessary guarantee of completion is wait for our particular command to
> have been consumed, and as long as we keep track of where it is there is
> no need to block other CPUs from adding further commands in the
> meantime. There is one theoretical (but incredibly unlikely) edge case
> to avoid, where cons has wrapped twice to still appear 'behind' the sync
> position - this is easily disambiguated by adding a generation count to
> the queue to indicate when prod wraps, since cons cannot wrap twice
> without prod having wrapped at least once.
> 
> Signed-off-by: Robin Murphy <robin.murphy-5wv7dgnIgG8@public.gmane.org>
> ---
> 
> v2: New
> 
>  drivers/iommu/arm-smmu-v3.c | 81 ++++++++++++++++++++++++++++++++-------------
>  1 file changed, 58 insertions(+), 23 deletions(-)
> 
> diff --git a/drivers/iommu/arm-smmu-v3.c b/drivers/iommu/arm-smmu-v3.c
> index 311f482b93d5..f5c5da553803 100644
> --- a/drivers/iommu/arm-smmu-v3.c
> +++ b/drivers/iommu/arm-smmu-v3.c
> @@ -417,7 +417,6 @@
>  
>  /* High-level queue structures */
>  #define ARM_SMMU_POLL_TIMEOUT_US	100
> -#define ARM_SMMU_CMDQ_DRAIN_TIMEOUT_US	1000000 /* 1s! */
>  #define ARM_SMMU_SYNC_TIMEOUT_US	1000000 /* 1s! */
>  
>  #define MSI_IOVA_BASE			0x8000000
> @@ -540,6 +539,7 @@ struct arm_smmu_queue {
>  struct arm_smmu_cmdq {
>  	struct arm_smmu_queue		q;
>  	spinlock_t			lock;
> +	int				generation;
>  };
>  
>  struct arm_smmu_evtq {
> @@ -770,21 +770,12 @@ static void queue_inc_prod(struct arm_smmu_queue *q)
>  	writel(q->prod, q->prod_reg);
>  }
>  
> -/*
> - * Wait for the SMMU to consume items. If drain is true, wait until the queue
> - * is empty. Otherwise, wait until there is at least one free slot.
> - */
> -static int queue_poll_cons(struct arm_smmu_queue *q, bool drain, bool wfe)
> +/* Wait for the SMMU to consume items, until there is at least one free slot */
> +static int queue_poll_cons(struct arm_smmu_queue *q, bool wfe)
>  {
> -	ktime_t timeout;
> -	unsigned int delay = 1;
> +	ktime_t timeout = ktime_add_us(ktime_get(), ARM_SMMU_POLL_TIMEOUT_US);
>  
> -	/* Wait longer if it's queue drain */
> -	timeout = ktime_add_us(ktime_get(), drain ?
> -					    ARM_SMMU_CMDQ_DRAIN_TIMEOUT_US :
> -					    ARM_SMMU_POLL_TIMEOUT_US);
> -
> -	while (queue_sync_cons(q), (drain ? !queue_empty(q) : queue_full(q))) {
> +	while (queue_sync_cons(q), queue_full(q)) {
>  		if (ktime_compare(ktime_get(), timeout) > 0)
>  			return -ETIMEDOUT;
>  
> @@ -792,8 +783,7 @@ static int queue_poll_cons(struct arm_smmu_queue *q, bool drain, bool wfe)
>  			wfe();
>  		} else {
>  			cpu_relax();
> -			udelay(delay);
> -			delay *= 2;
> +			udelay(1);
>  		}
>  	}
>  
> @@ -959,15 +949,20 @@ static void arm_smmu_cmdq_skip_err(struct arm_smmu_device *smmu)
>  	queue_write(Q_ENT(q, cons), cmd, q->ent_dwords);
>  }
>  
> -static void arm_smmu_cmdq_insert_cmd(struct arm_smmu_device *smmu, u64 *cmd)
> +static u32 arm_smmu_cmdq_insert_cmd(struct arm_smmu_device *smmu, u64 *cmd)
>  {
>  	struct arm_smmu_queue *q = &smmu->cmdq.q;
>  	bool wfe = !!(smmu->features & ARM_SMMU_FEAT_SEV);
>  
>  	while (queue_insert_raw(q, cmd) == -ENOSPC) {
> -		if (queue_poll_cons(q, false, wfe))
> +		if (queue_poll_cons(q, wfe))
>  			dev_err_ratelimited(smmu->dev, "CMDQ timeout\n");
>  	}
> +
> +	if (Q_IDX(q, q->prod) == 0)
> +		smmu->cmdq.generation++;

The readers of generation are using READ_ONCE, so you're missing something
here.

> +
> +	return q->prod;
>  }
>  
>  static void arm_smmu_cmdq_issue_cmd(struct arm_smmu_device *smmu,
> @@ -997,15 +992,54 @@ static int arm_smmu_sync_poll_msi(struct arm_smmu_device *smmu, u32 sync_idx)
>  	return (int)(val - sync_idx) < 0 ? -ETIMEDOUT : 0;
>  }
>  
> +static int arm_smmu_sync_poll_cons(struct arm_smmu_device *smmu, u32 sync_idx,
> +				   int sync_gen)
> +{
> +	ktime_t timeout = ktime_add_us(ktime_get(), ARM_SMMU_SYNC_TIMEOUT_US);
> +	struct arm_smmu_queue *q = &smmu->cmdq.q;
> +	bool wfe = !!(smmu->features & ARM_SMMU_FEAT_SEV);
> +	unsigned int delay = 1;
> +
> +	do {
> +		queue_sync_cons(q);
> +		/*
> +		 * If we see updates quickly enough, cons has passed sync_idx,
> +		 * but not yet wrapped. At worst, cons might have actually
> +		 * wrapped an even number of times, but that still guarantees
> +		 * the original sync must have been consumed.
> +		 */
> +		if (Q_IDX(q, q->cons) >= Q_IDX(q, sync_idx) &&
> +		    Q_WRP(q, sync_idx) == Q_WRP(q, q->cons))

Can you move this into a separate function please, like we have for
queue_full and queue_empty?

> +			return 0;
> +		/*
> +		 * Otherwise, cons may have passed sync_idx and wrapped one or
> +		 * more times to appear behind it again, but in that case prod
> +		 * must also be one or more generations ahead.
> +		 */
> +		if (Q_IDX(q, q->cons) < Q_IDX(q, sync_idx) &&
> +		    READ_ONCE(smmu->cmdq.generation) != sync_gen)

There's another daft overflow case here which deserves a comment (and even
if it *did* happen, we'll recover gracefully).

> +			return 0;
> +
> +		if (wfe) {
> +			wfe();

This is a bit scary... if we miss a generation update, just due to store
propagation latency (maybe it's buffered by the updater), I think we can
end up going into wfe when there's not an event pending. Using xchg
everywhere might work, but there's still a race on having somebody update
generation. The ordering here just looks generally suspicious to me because
you have the generation writer in arm_smmu_cmdq_insert_cmd effectively
doing:

  Write prod
  Write generation

and the reader in arm_smmu_sync_poll_cons doing:

  Read cons
  Read generation

so I can't see anything that gives you order to guarantee that the
generation is seen to be up-to-date.

But it's also Friday evening and my brain is pretty fried ;)

Will