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

From: Fenghua Yu fenghua.yu@intel.com 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.