linux-kernel - Re: [PATCH v2 -mm 0/2] x86: per-device dma_mapping

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Date:	Mon, 26 May 2008 09:39:20 +0530
From:	Amit Shah <amit.shah@...ranet.com>
To:	Muli Ben-Yehuda <muli@...ibm.com>
Cc:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	alexisb@...ibm.com, andi@...stfloor.org, kvm@...r.kernel.org,
	avi@...ranet.com
Subject: Re: [PATCH v2 -mm 0/2] x86: per-device dma_mapping_ops

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.

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.

Does this work?
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