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

[Baolu Lu]

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