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    SubjectRe: [PATCH 0/9][RFC] stackable dma_ops for x86
    On Mon, Sep 29, 2008 at 10:42:37PM +0900, FUJITA Tomonori wrote:
    > On Mon, 29 Sep 2008 15:26:47 +0200
    > Joerg Roedel <joerg.roedel@amd.com> wrote:
    >
    > > On Mon, Sep 29, 2008 at 10:16:39PM +0900, FUJITA Tomonori wrote:
    > > > On Sun, 28 Sep 2008 20:49:26 +0200
    > > > Joerg Roedel <joro@8bytes.org> wrote:
    > > >
    > > > > On Sun, Sep 28, 2008 at 11:21:26PM +0900, FUJITA Tomonori wrote:
    > > > > > On Mon, 22 Sep 2008 20:21:12 +0200
    > > > > > Joerg Roedel <joerg.roedel@amd.com> wrote:
    > > > > >
    > > > > > > Hi,
    > > > > > >
    > > > > > > this patch series implements stackable dma_ops on x86. This is useful to
    > > > > > > be able to fall back to a different dma_ops implementation if one can
    > > > > > > not handle a particular device (as necessary for example with
    > > > > > > paravirtualized device passthrough or if a hardware IOMMU only handles a
    > > > > > > subset of available devices).
    > > > > >
    > > > > > We already handle the latter. This patchset is more flexible but
    > > > > > seems to incur more overheads.
    > > > > >
    > > > > > This feature will be used for only paravirtualized device passthrough?
    > > > > > If so, I feel that there is more simpler (and specific) solutions for
    > > > > > it.
    > > > >
    > > > > Its not only for device passthrough. It handles also the cases where a
    > > > > hardware IOMMU does not handle all devices in the system (like in some
    > > > > Calgary systems but also possible with AMD IOMMU). With this patchset we
    > > >
    > > > I know that. As I wrote in the previous mail, we already solved that
    > > > problem with per-device-dma-ops.
    > > >
    > > > My question is what unsolved problems this patchset can fix?
    > > >
    > > >
    > > > This patchset is named "stackable dma_ops" but it's different from
    > > > what we discussed as "stackable dma_ops". This patchset provides
    > > > IOMMUs a generic mechanism to set up "stackable dma_ops". But this
    > > > patchset doesn't solve the problem that a hardware IOMMU does not
    > > > handle all devices (it was already solved with per-device-dma-ops).
    > > >
    > > > If paravirtualized device passthrough still needs to call multiple
    > > > dma_ops, then this patchset doesn't solve that issue.
    > >
    > > Ok, the name "stackable" is misleading and was a bad choice. I will
    > > rename it to "multiplexing". This should make it more clear what it is.
    > > Like you pointed out, the problems are solved with per-device dma_ops,
    > > but in the current implementation it needs special hacks in the IOMMU
    > > drivers to use these per-device dma_ops.
    > > I see this patchset as a continuation of the per-device dma_ops idea. It
    > > moves the per-device handling out of the specific drivers to a common
    > > place. So we can avoid or remove special hacks in the IOMMU drivers.
    >
    > Basically, I'm not against this patchset. It simplify Calgary and AMD
    > IOMMUs code to set up per-device-dma-ops (though it makes dma_ops a
    > bit complicated).

    Yes. But mind that this patchset adds complexity to one point (at
    dma_ops initialization) while we can avoid and remove it
    at many other places (in the dma_ops drivers).

    > But it doesn't solve any problems including the paravirtualized device
    > passthrough. When I wrote per-device-dma-ops, I expected that KVM
    > people want more changes (such as stackable dma_ops) to dma_ops for
    > the paravirtualized device passthrough. I'd like to hear what they
    > want first.

    Sure. Therefore this patchset is RFC and I cc'ed them.

    Joerg

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