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Date:   Wed, 22 Nov 2023 09:25:42 -0400
From:   Jason Gunthorpe <jgg@nvidia.com>
To:     "Tian, Kevin" <kevin.tian@intel.com>
Cc:     "Liu, Yi L" <yi.l.liu@intel.com>,
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        "Zeng, Xin" <xin.zeng@intel.com>,
        "Zhao, Yan Y" <yan.y.zhao@intel.com>
Subject: Re: [PATCH v7 1/3] iommufd: Add data structure for Intel VT-d
 stage-1 cache invalidation
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On Wed, Nov 22, 2023 at 04:58:24AM +0000, Tian, Kevin wrote:

> As Yi/Baolu discussed there is an issue in intel-iommu driver which
> incorrectly skips devtlb invalidation in the guest with the assumption
> that the host combines iotlb/devtlb invalidation together. This is
> incorrect and should be fixed.

Yes, this seems quite problematic - you guys will have to think of
something and decide what kind of backward compat you want :(

Maybe the viommu driver can observe the guest and if it sees an ATC
invalidation assume it is non-buggy, until seen it can do a combined
flush.

> But what I was talking about earlier is about the uAPI between
> viommu and iommu driver. I don't see a need of having separate
> invalidation cmds for each, as I'm not sure what the user can
> expect in the window when iotlb and devtlb are out of sync.

If the guest is always issuing the device invalidation then I don't
see too much point in suppressing it in the kernel. Just forward it
naturally.

> then we just define hwpt 'cache' invalidation in vtd always refers to
> both iotlb and devtlb. Then viommu just needs to call invalidation
> uapi once when emulating virtual iotlb invalidation descriptor
> while emulating the following devtlb invalidation descriptor
> as a nop.

In principle ATC and IOMMU TLB invalidations should not always be
linked.

Any scenario that allows devices to share an IOTLB cache tag requires
fewer IOMMU TLB invalidations than ATC invalidations.

I like the view of this invalidation interface as reflecting the
actual HW and not trying to be smarter an real HW.

I'm fully expecting that Intel will adopt an direct-DMA flush queue
like SMMU and AMD have already done as a performance optimization. In
this world it makes no sense that the behavior of the direct DMA queue
and driver mediated queue would be different.

Jason