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Re: [Qemu-devel] RFC [v2]: vfio / device assignment -- layout of device

From: Scott Wood
Subject: Re: [Qemu-devel] RFC [v2]: vfio / device assignment -- layout of device fd files
Date: Tue, 27 Sep 2011 16:28:27 -0500
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On 09/26/2011 07:45 PM, Alex Williamson wrote:
> On Mon, 2011-09-26 at 18:59 -0500, Scott Wood wrote:
>> On 09/26/2011 01:34 PM, Alex Williamson wrote:
>>> /* Reset the device */
>>> #define VFIO_DEVICE_RESET                   _IO(, ,)
>> What generic way do we have to do this?  We should probably have a way
>> to determine whether it's possible, without actually asking to do it.
> It's not generic, it could be a VFIO_DEVICE_PCI_RESET or we could add a
> bit to the device flags to indicate if it's available or we could add a
> "probe" arg to the ioctl to either check for existence or do it.

Even with PCI, isn't this only possible if function-level reset is
supported?  I think we need a flag.

For devices that can't be reset by the kernel, we'll want the ability to
stop/start DMA acccess through the IOMMU (or other bus-specific means),
separate from whether the fd is open.  If a device is assigned to a
partition and that partition gets reset, we'll want to disable DMA
before we re-use the memory, and enable it after the partition has reset
or quiesced the device (which requires the fd to be open).

>>> /* PCI MSI setup, arg[0] = #, arg[1-n] = eventfds */
>>> #define VFIO_DEVICE_PCI_SET_MSI_EVENTFDS    _IOW(, , int)
>>> Hope that covers it.
>> It could be done this way, but I predict that the code (both kernel and
>> user side) will be larger.  Maybe not much more complex, but more
>> boilerplate.
>> How will you manage extensions to the interface?
> I would assume we'd do something similar to the kvm capabilities checks.

This information is already built into the data-structure approach.

>> The table should not be particularly large, and you'll need to keep the
>> information around in some form regardless.  Maybe in the PCI case you
>> could produce it dynamically (though I probably wouldn't), but it really
>> wouldn't make sense in the device tree case.
> It would be entirely dynamic for PCI, there's no advantage to caching
> it.  Even for device tree, if you can't fetch it dynamically, you'd have
> to duplicate it between an internal data structure and a buffer reading
> the table.

I don't think we'd need to keep the device tree path/index info around
for anything but the table -- but really, this is a minor consideration.

>> You also lose the ability to easily have a human look at the hexdump for
>> debugging; you'll need a special "lsvfio" tool.  You might want one
>> anyway to pretty-print the info, but with ioctls it's mandatory.
> I don't think this alone justifies duplicating the data and making it
> difficult to parse on both ends.  Chances are we won't need such a tool
> for the ioctl interface because it's easier to get it right the first
> time ;)

It's not just useful for getting the code right, but for e.g. sanity
checking that the devices were bound properly.  I think such a tool
would be generally useful, no matter what the kernel interface ends up
being.  I don't just use lspci to debug the PCI subsystem. :-)

> Note that I'm not stuck on this interface, I was just thinking about how
> to generate the table last week, it seemed like a pain so I thought I'd
> spend a few minutes outlining an ioctl interface... turns out it's not
> so bad.  Thanks,

Yeah, it can work either way, as long as the information's there and
there's a way to add new bits of information, or new bus types, down the
road.  Mainly a matter of aesthetics between the two.


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