Re: [PATCH v3 4/5] iommu/arm-smmu-v3: Add host support for NVIDIA Grace CMDQ-V

[Robin Murphy <robin.murphy@arm.com>]

On 2021-11-19 07:19, Nicolin Chen wrote:
> From: Nate Watterson <nwatterson@nvidia.com>
> 
> NVIDIA's Grace Soc has a CMDQ-Virtualization (CMDQV) hardware,
> which extends the standard ARM SMMU v3 IP to support multiple
> VCMDQs with virtualization capabilities. In-kernel of host OS,
> they're used to reduce contention on a single queue. In terms
> of command queue, they are very like the standard CMDQ/ECMDQs,
> but only support CS_NONE in the CS field of CMD_SYNC command.
> 
> This patch adds a new nvidia-grace-cmdqv file and inserts its
> structure pointer into the existing arm_smmu_device, and then
> adds related function calls in the arm-smmu-v3 driver.
> 
> In the CMDQV driver itself, this patch only adds minimal part
> for host kernel support. Upon probe(), VINTF0 is reserved for
> in-kernel use. And some of the VCMDQs are assigned to VINTF0.
> Then the driver will select one of VCMDQs in the VINTF0 based
> on the CPU currently executing, to issue commands.

Is there a tangible difference to DMA API or VFIO performance?

[...]
> +struct arm_smmu_cmdq *nvidia_grace_cmdqv_get_cmdq(struct arm_smmu_device *smmu)
> +{
> +	struct nvidia_grace_cmdqv *cmdqv = smmu->nvidia_grace_cmdqv;
> +	struct nvidia_grace_cmdqv_vintf *vintf0 = &cmdqv->vintf0;
> +	u16 qidx;
> +
> +	/* Check error status of vintf0 */
> +	if (!FIELD_GET(VINTF_STATUS, vintf0->status))
> +		return &smmu->cmdq;
> +
> +	/*
> +	 * Select a vcmdq to use. Here we use a temporal solution to
> +	 * balance out traffic on cmdq issuing: each cmdq has its own
> +	 * lock, if all cpus issue cmdlist using the same cmdq, only
> +	 * one CPU at a time can enter the process, while the others
> +	 * will be spinning at the same lock.
> +	 */
> +	qidx = smp_processor_id() % cmdqv->num_vcmdqs_per_vintf;

How does ordering work between queues? Do they follow a global order 
such that a sync on any queue is guaranteed to complete all prior 
commands on all queues?

The challenge to make ECMDQ useful to Linux is how to make sure that all 
the commands expected to be within scope of a future CMND_SYNC plus that 
sync itself all get issued on the same queue, so I'd be mildly surprised 
if you didn't have the same problem.

Robin.

> +	return &vintf0->vcmdqs[qidx];
> +}