RE: [PATCH v2] dmaengine: idxd: Do not enable user type Work Queue without Shared Virtual Addressing

[Tian, Kevin]

> From: Fenghua Yu <fenghua.yu@xxxxxxxxx>
> Sent: Saturday, October 15, 2022 6:26 AM
> 
> +	/*
> +	 * User type WQ is enabled only when SVA is enabled for two
> reasons:
> +	 *   - If no IOMMU or IOMMU Passthrough without SVA, userspace

This statement is kind of misleading. Even if IOMMU is in DMA mode
user type WQ still doesn't work w/o SVA. 

> +	 *     can directly access physical address through the WQ.
> +	 *   - The IDXD cdev driver does not provide any ways to pin
> +	 *     user pages and translate the address from user VA to IOVA or
> +	 *     PA without IOMMU SVA. Therefore the application has no way
> +	 *     to instruct the device to perform DMA function. This makes
> +	 *     the cdev not usable for normal application usage.
> +	 */

It could be simply stated as "SVA is the only secure/reliable way for
the device to access user space memory"

> +	if (!device_user_pasid_enabled(idxd)) {
> +		idxd->cmd_status = IDXD_SCMD_WQ_USER_NO_IOMMU;

be specific i.e. IDXD_SCMD_WQ_USER_NO_SVA

> +		dev_dbg(&idxd->pdev->dev,
> +			"User type WQ cannot be enabled without SVA.\n");
> +
> +		return -EOPNOTSUPP;
> +	}
> +
>  

with above change the check on pasid_enabled should be removed from
idxd_cdev_open() and idxd_cdev_release(). They should always work
with SVA enabled.