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[Qemu-devel] Re: [PATCHv2 10/12] tap: add vhost/vhostfd options
From: |
Michael S. Tsirkin |
Subject: |
[Qemu-devel] Re: [PATCHv2 10/12] tap: add vhost/vhostfd options |
Date: |
Mon, 1 Mar 2010 21:27:32 +0200 |
User-agent: |
Mutt/1.5.19 (2009-01-05) |
On Sun, Feb 28, 2010 at 10:39:21PM +0000, Paul Brook wrote:
> > I'm sympathetic to your arguments though. As qemu is today, the above
> > is definitely the right thing to do. But ram is always ram and ram
> > always has a fixed (albeit non-linear) mapping within a guest.
>
> I think this assumption is unsafe. There are machines where RAM mappings can
> change. It's not uncommon for a chip select (i.e. physical memory address
> region) to be switchable to several different sources, one of which may be
> RAM. I'm pretty sure this functionality is present (but not actually
> implemented) on some of the current qemu targets.
>
> I agree that changing RAM mappings under an active DMA is a fairly suspect
> thing to do. However I think we need to avoid cache mappings between separate
> DMA transactions i.e. when the guest can know that no DMA will occur, and
> safely remap things.
>
> I'm also of the opinion that virtio devices should behave the same as any
> other device. i.e. if you put a virtio-net-pci device on a PCI bus behind an
> IOMMU, then it should see the same address space as any other PCI device in
> that location.
It already doesn't. virtio passes physical memory addresses
to device instead of DMA addresses.
> Apart from anything else, failure to do this breaks nested
> virtualization.
Assigning PV device in nested virtualization? It could work, but not
sure what the point would be.
> While qemu doesn't currently implement an IOMMU, the DMA
> interfaces have been designed to allow it.
>
> > void cpu_ram_add(target_phys_addr_t start, ram_addr_t size);
>
> We need to support aliased memory regions. For example the ARM RealView
> boards
> expose the first 256M RAM at both address 0x0 and 0x70000000. It's also
> common
> for systems to create aliases by ignoring certain address bits. e.g. each sim
> slot is allocated a fixed 256M region. Populating that slot with a 128M stick
> will cause the contents to be aliased in both the top and bottom halves of
> that region.
>
> Paul
- [Qemu-devel] Re: [PATCHv2 10/12] tap: add vhost/vhostfd options,
Michael S. Tsirkin <=
- [Qemu-devel] Re: [PATCHv2 10/12] tap: add vhost/vhostfd options, Anthony Liguori, 2010/03/02
- [Qemu-devel] Re: [PATCHv2 10/12] tap: add vhost/vhostfd options, Paul Brook, 2010/03/02
- [Qemu-devel] Re: [PATCHv2 10/12] tap: add vhost/vhostfd options, Anthony Liguori, 2010/03/02
- [Qemu-devel] Re: [PATCHv2 10/12] tap: add vhost/vhostfd options, Paul Brook, 2010/03/02
- [Qemu-devel] Re: [PATCHv2 10/12] tap: add vhost/vhostfd options, Anthony Liguori, 2010/03/02
- [Qemu-devel] Re: [PATCHv2 10/12] tap: add vhost/vhostfd options, Paul Brook, 2010/03/02
- [Qemu-devel] Re: [PATCHv2 10/12] tap: add vhost/vhostfd options, Michael S. Tsirkin, 2010/03/02
- [Qemu-devel] Re: [PATCHv2 10/12] tap: add vhost/vhostfd options, Anthony Liguori, 2010/03/02