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

[Anthony Liguori <aliguori@us.ibm.com>]

Peter Maydell <peter.maydell@linaro.org> writes:

> On 8 July 2013 14:45, Alexander Graf <agraf@suse.de> 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.

Regards,

Anthony Liguori

>
> -- PMM