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Re: [Qemu-devel] [PATCH 0/8] Add virtio-mmio and use it in vexpress


From: Alexander Graf
Subject: Re: [Qemu-devel] [PATCH 0/8] Add virtio-mmio and use it in vexpress
Date: Mon, 8 Jul 2013 22:47:46 +0200

On 08.07.2013, at 22:08, Anthony Liguori wrote:

> Peter Maydell <address@hidden> writes:
> 
>> On 8 July 2013 14:45, Alexander Graf <address@hidden> wrote:
>>> On 08.07.2013, at 15:23, Peter Maydell wrote:
>>>> Now I'm completely confused. Why would assigned devices
>>>> have anything to do with this? Can you explain in more
>>>> detail, because I don't really see what you're suggesting?
>>> 
>>> The only missing link we have to create any device using -device
>>> on the command line is the IRQ line enumeration. If we can allocate
>>> IRQ lines automatically, we can put any command line given -device
>>> onto our main system bus that is non-pci, non-isa.
>> 
>> If the user is expected to be able to get the MMIO address
>> right (which they'd have to specify on the command line
>> somehow too) why not require them to specify the IRQ number
>> while they're doing it? I'm a bit suspicious of anything
>> that requires the user to specify to that level of detail
>> though, since it requires a lot of inside knowledge about the
>> board.
>> 
>> This is the whole reason for having the separate transport:
>> the board gets to take care of the board specific detail of
>> how to wire up the transport, and the user just asks to
>> create the backends that plug automatically into it.
>> The virtio command line options are complicated and confusing
>> enough as it is.
>> 
>>> So if we want to ever support VFIO for platform devices,
>>> the user will want to pass -device vfio-ahci,foo=bar on
>>> the command line to assign an AHCI device.
>> 
>> This appears to be seriously short on actually specifying
>> enough information to wire a device up.
>> 
>>> The only infrastructure blocker we have for that today
>>> is the IRQ allocation.
>> 
>> DMA lines? Specifying the right location in the address space?
>> 
>>> Maybe we could even try to be as smart as putting the MMIO
>>> regions into guest address space intelligently automatically.
>> 
>> This sounds likely to cause problems with migration unless
>> we can guarantee that we always pick the same place.
> 
> I think we're trying to fit a square peg into a round hole.
> 
> virtio-mmio is a virtio transport where each device has a dedicated set
> of system resources.
> 
> Alex, it sounds like you want virtio-mmio-bus which would be a single
> set of system resources that implemented a virtio bus on top of it.

Well, what I really want is a sysbus that behaves like PCI from a usability 
point of view ;).


Alex




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