Re: [PATCH v3 07/10] iommu/vt-d: Add iotlb flush for nested domain

[Baolu Lu <baolu.lu@linux.intel.com>]

On 2023/6/8 15:14, Liu, Yi L wrote:
>>> + *                                              stage-1 page table cache
>>> + *                                              invalidation
>>> + * @IOMMU_VTD_QI_FLAGS_LEAF: The LEAF flag indicates whether only the
>>> + *                           leaf PTE caching needs to be invalidated
>>> + *                           and other paging structure caches can be
>>> + *                           preserved.
>>> + */
>> what about "Drain Reads" and "Drain Writes"? Is the user allowed/required
>> to provide those hints?
> All other comments got. For these two hints, the two flags are from the IOTLB
> Invalidation descriptor. Per below description, the hardware that supports nested
> should support drain and does not require software to ask for it. So it appears no
> need to define them in uapi.
> 
> "Hardware implementation with Major Version 2 or higher (VER_REG),
> always performs required drain without software explicitly requesting
> a drain in IOTLB invalidation. This field is deprecated and hardware
> will always report it as 1 to maintain backward compatibility with
> software"

Make sense. Perhaps we can also remove below code in
__iommu_flush_iotlb():

         /* Note: set drain read/write */
#if 0
         /*
          * This is probably to be super secure.. Looks like we can
          * ignore it without any impact.
          */
         if (cap_read_drain(iommu->cap))
                 val |= DMA_TLB_READ_DRAIN;
#endif

Best regards,
baolu