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Re: [Qemu-devel] [PATCH 1/9] Add stub functions for PCI device models to
From: |
David Gibson |
Subject: |
Re: [Qemu-devel] [PATCH 1/9] Add stub functions for PCI device models to do PCI DMA |
Date: |
Tue, 18 Oct 2011 12:46:50 +1100 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
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.
> 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.
> 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.
--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson
- 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, Michael S. Tsirkin, 2011/10/02
- Re: [Qemu-devel] [PATCH 1/9] Add stub functions for PCI device models to do PCI DMA, Avi Kivity, 2011/10/02
- Re: [Qemu-devel] [PATCH 1/9] Add stub functions for PCI device models to do PCI DMA, Michael S. Tsirkin, 2011/10/02
- Re: [Qemu-devel] [PATCH 1/9] Add stub functions for PCI device models to do PCI DMA, Avi Kivity, 2011/10/02
- Re: [Qemu-devel] [PATCH 1/9] Add stub functions for PCI device models to do PCI DMA, Anthony Liguori, 2011/10/03
- 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, 2011/10/13
- 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 <=
- 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, 2011/10/19
- 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