Messages in this thread | | | Date | Mon, 29 Sep 2008 15:51:27 +0200 | From | Joerg Roedel <> | Subject | Re: [PATCH 0/9][RFC] stackable dma_ops for x86 |
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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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