Messages in this thread | | | Subject | Re: [PATCH v2 -mm 0/2] x86: per-device dma_mapping_ops | From | FUJITA Tomonori <> | Date | Tue, 27 May 2008 08:50:54 +0900 |
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On Mon, 26 May 2008 22:14:34 +0530 Amit Shah <amit.shah@qumranet.com> wrote:
> On Monday 26 May 2008 11:41:52 FUJITA Tomonori wrote: > > On Mon, 26 May 2008 09:39:20 +0530 > > > > Amit Shah <amit.shah@qumranet.com> wrote: > > > On Sunday 25 May 2008 12:50:11 Muli Ben-Yehuda wrote: > > > > On Thu, May 22, 2008 at 04:13:02PM +0530, Amit Shah wrote: > > > > > OK; this sounds helpful. the hook can make a hypercall and confirm > > > > > with the host kernel if the device in question is an assigned > > > > > physical device. If yes, we replace the dma_ops. Though, the > > > > > original intent of having stackable ops is that we might want to go > > > > > through the swiotlb in the guest even for an assigned device if the > > > > > guest dma addresses are not in the addressable range of the guest > > > > > chipset. > > > > > > > > > > > created (it works with hot plugging). It enables IOMMUs to set up > > > > > > an appropriate dma_mapping_ops per device. > > > > > > > > > > From what we've discussed so far, it looks like stackable dma ops > > > > > will definitely be needed. Does this patchset provide something that > > > > > stacking won't? > > > > > > > > Yes---this patchset let's you have a per-device dma-ops, whereas with > > > > stackable you only get global dma-ops. I think it's clear we need > > > > both, and I think per-device dma-ops are the first thing that's > > > > needed. Stacking can then be introduced on a per-device basis. > > > > > > When we would want stacking, we'll want it globally and not per-device, > > > isn't it? Or at least for devices on a particular bus. > > > > > > When an IOMMU driver registers itself, it should tell which devices it's > > > interested in (each device behind a bus or by enumerating each device it > > > cares for). This should take care of all the scenarios and we won't have > > > the need for per-device dma_ops. > > > > Well, without per-device dma_ops, IOMMUs could live. But it's pretty > > hacky. Every time a dma operation is called, IOMMUs need to figure out > > how a device should be handled. > > What if this information could be hidden behind (a slightly complicated) > get_dma_ops()? Also, each of the operations in dma_ops will see if there's > something else down the stack that might be interested in the current device.
dma_ops can't do anything since only IOMMUs know what to do against a device.
Whatever you implement in dma_ops, without per-device dma_ops, IOMMUs need to figure out what to do a device every time a dma operation is called.
> My contention is that we are going to need stackable ops, and a full-fledged > stackable implementation is going to solve this problem as well. However, > this current implementation of per-device dma_ops looks like a really simple > and non-intrusive solution to one problem, that of getting rid of some > overheads in the IOMMU code.
I don't think that stackable ops solve the problem that some IOMMUs have.
> > If IOMMUs can set dma_ops for the device when a new device is created, > > IOMMUs don't care anything any more. That's much clean. That's What > > the POWER architecture does. > > > > > For something like pvdma, we can walk through the list of pci devices and > > > make a hypercall for each of them to get this information and have the > > > pvdma version of dma_ops registered for that device. This sounds like > > > it's per-device dma_ops, but it's not -- internally, the dma operations > > > walk through each of the IOMMUs registered and call them in sequence. > > > > As Muli poinsted out, For pvdma, you need stacking per-device > > dma_ops. With per-device dma_ops, you don't need hack like adding > > is_pv_device hook in dma_ops. You can set your dma_ops to only pci > > devices that you are interested. > > The hack was added only because there's no stackable dma api we have now. > Sure, per-device dma_ops is going to solve this problem and I like it. I'm > only saying we're also going to need stacking ops and in effect, per-device > dma_ops would just be replaced by them once we get the complete solution.
Again, stackable ops can't cleanly solve the problem that per-device dma_ops tries to solve. For example, you stack dma_ops like pvdma->hardare->nommu/swiotlb. How can pvdma_ops know if pvdma_ops needs to handle a device or not? pvdma_ops needs to skip some devices and handle some. per-device dma_ops enables us not to stack pvdma_ops for devices that pvdma_ops are not instrested in. That's much clean.
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