linux-kernel - Re: [PATCH v1 07/14] vfio: Add a device notifier interface

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Date:   Mon, 22 Mar 2021 15:16:16 +0000
From:   Christoph Hellwig <hch@...radead.org>
To:     Alex Williamson <alex.williamson@...hat.com>
Cc:     Christoph Hellwig <hch@...radead.org>, cohuck@...hat.com,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org, jgg@...dia.com,
        peterx@...hat.com
Subject: Re: [PATCH v1 07/14] vfio: Add a device notifier interface

On Fri, Mar 19, 2021 at 04:25:40PM -0600, Alex Williamson wrote:
> I've been trying to figure out how, but I think not.  A user can have
> multiple devices, each with entirely separate IOMMU contexts.  For each
> device, the user can create an mmap of memory to that device and add it
> to every other IOMMU context.  That enables peer to peer DMA between
> all the devices, across all the IOMMU contexts.  But each individual
> device has no direct reference to any IOMMU context other than its own.
> A callback on the IOMMU can't reach those other contexts either, there's
> no guarantee those other contexts are necessarily managed via the same
> vfio IOMMU backend driver.  A notifier is the best I can come up with,
> please suggest if you have other ideas.  Thanks,

Indeed, reviewing the set again it seems like we need the notifier
here.