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    SubjectRe: [RFC 10/20] iommu/iommufd: Add IOMMU_DEVICE_GET_INFO
    On Thu, Sep 30, 2021 at 08:49:03AM +0000, Tian, Kevin wrote:
    > > From: Jason Gunthorpe <jgg@nvidia.com>
    > > Sent: Thursday, September 23, 2021 8:22 PM
    > >
    > > > > These are different things and need different bits. Since the ARM path
    > > > > has a lot more code supporting it, I'd suggest Intel should change
    > > > > their code to use IOMMU_BLOCK_NO_SNOOP and abandon
    > > IOMMU_CACHE.
    > > >
    > > > I didn't fully get this point. The end result is same, i.e. making the DMA
    > > > cache-coherent when IOMMU_CACHE is set. Or if you help define the
    > > > behavior of IOMMU_CACHE, what will you define now?
    > >
    > > It is clearly specifying how the kernel API works:
    > >
    > > !IOMMU_CACHE
    > > must call arch cache flushers
    > > IOMMU_CACHE -
    > > do not call arch cache flushers
    > > IOMMU_CACHE|IOMMU_BLOCK_NO_SNOOP -
    > > dot not arch cache flushers, and ignore the no snoop bit.
    >
    > Who will set IOMMU_BLOCK_NO_SNOOP?

    Basically only qemu due to specialized x86 hypervisor knowledge.

    The only purpose of this attribute is to support a specific
    virtualization use case where a whole bunch of stuff is broken
    together:
    - the cache maintenance instructions are not available to a guest
    - the guest isn't aware that the instructions don't work and tells
    the device to issue no-snoop TLPs
    - The device ignores the 'disable no-snoop' flag in the PCIe config
    space

    Thus things become broken.

    Jason

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