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Re: [Qemu-devel] [PATCH 1/9] Add stub functions for PCI device models to
From: |
Avi Kivity |
Subject: |
Re: [Qemu-devel] [PATCH 1/9] Add stub functions for PCI device models to do PCI DMA |
Date: |
Wed, 19 Oct 2011 11:10:15 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:7.0.1) Gecko/20110930 Thunderbird/7.0.1 |
On 10/18/2011 03:46 AM, David Gibson wrote:
> On Sun, Oct 16, 2011 at 03:15:53PM +0200, Avi Kivity wrote:
> > On 10/14/2011 04:14 AM, David Gibson wrote:
> > > > Virtio is a very, very special case. virtio requires coherent RAM
> > > > access.
> > >
> > > Right. Virtio's access to memory is *not* emulated PCI DMA, it's
> > > god-like hypervisor access to guest system memory. It should
> > > correctly bypass any IOMMU, and so should remain as
> > > cpu_physical_memory_rw() or the atomic accessors, rather than being
> > > converted to this new API.
> >
> > virtio should definitely not bypass an iommu.
>
> So, I just had a chat with Rusty about this. Perhaps it shouldn't,
> but it does. The spec is in terms of guest physical addresses, not
> bus/DMA addresses, and more to the point the Linux driver does *not*
> do the necessary dma_map() and unmap operations to treat this as a PCI
> DMA. So like it or not, god-like hypervisor access rather than
> emulated PCI DMA is what it does.
Wow, how did we manage to break virtio in so many different ways?
Is there a way to unbreak it? On x86 it will continue to work if we
rewrite the spec in terms of pci dma, what about non-x86?
>
> > A guest may assign a
> > virtio device to nested guests, and would wish it confined by the
> > emulated iommu.
>
> Well, that would be nice, but it can't be done. It could be fixed,
> but it would be an incompatible change so it would need a new feature
> bit corresponding changes in the Linux driver to do the dma map/unmap
> if it accepts the "respect IOMMU" feature.
Needs to be done IMO.
>
> > More generally, a guest sees a virtio device as just another pci device,
> > and has no way to tell that it bypasses the iommu.
>
> Well, except the fact that the driver knows its a virtio device,
> because it's a virtio driver. It's not like you can write a driver
> that uses PCI DMA without knowing the particulars of the device you're
> using.
virtio-pci knows it's pci, there's no excuse.
--
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.
- Re: [Qemu-devel] [PATCH 1/9] Add stub functions for PCI device models to do PCI DMA, (continued)
- Re: [Qemu-devel] [PATCH 1/9] Add stub functions for PCI device models to do PCI DMA, David Gibson, 2011/10/13
- Re: [Qemu-devel] [PATCH 1/9] Add stub functions for PCI device models to do PCI DMA, Michael S. Tsirkin, 2011/10/16
- Re: [Qemu-devel] [PATCH 1/9] Add stub functions for PCI device models to do PCI DMA, Avi Kivity, 2011/10/16
- Re: [Qemu-devel] [PATCH 1/9] Add stub functions for PCI device models to do PCI DMA, David Gibson, 2011/10/17
- Re: [Qemu-devel] [PATCH 1/9] Add stub functions for PCI device models to do PCI DMA, Michael S. Tsirkin, 2011/10/18
- Re: [Qemu-devel] [PATCH 1/9] Add stub functions for PCI device models to do PCI DMA, Rusty Russell, 2011/10/18
- Re: [Qemu-devel] [PATCH 1/9] Add stub functions for PCI device models to do PCI DMA,
Avi Kivity <=
- Re: [Qemu-devel] [PATCH 1/9] Add stub functions for PCI device models to do PCI DMA, David Gibson, 2011/10/19
- Re: [Qemu-devel] [PATCH 1/9] Add stub functions for PCI device models to do PCI DMA, David Gibson, 2011/10/11
- Re: [Qemu-devel] [PATCH 1/9] Add stub functions for PCI device models to do PCI DMA, Michael S. Tsirkin, 2011/10/12
- Re: [Qemu-devel] [PATCH 1/9] Add stub functions for PCI device models to do PCI DMA, David Gibson, 2011/10/12
- Re: [Qemu-devel] [PATCH 1/9] Add stub functions for PCI device models to do PCI DMA, David Gibson, 2011/10/12
- Re: [Qemu-devel] [PATCH 1/9] Add stub functions for PCI device models to do PCI DMA, David Gibson, 2011/10/11
- Re: [Qemu-devel] [PATCH 1/9] Add stub functions for PCI device models to do PCI DMA, Michael S. Tsirkin, 2011/10/12
- Re: [Qemu-devel] [PATCH 1/9] Add stub functions for PCI device models to do PCI DMA, Gerd Hoffmann, 2011/10/12
Re: [Qemu-devel] [PATCH 1/9] Add stub functions for PCI device models to do PCI DMA, Anthony Liguori, 2011/10/03