Re: [Qemu-devel] [RESEND RFC 0/6] AMD XGBE KVM platform passthrough

[Christoffer Dall]

On Thu, Nov 19, 2015 at 11:44 PM, Alex Williamson
<address@hidden> wrote:
> On Thu, 2015-11-19 at 15:22 +0000, Eric Auger wrote:
>> I am resending this RFC from Oct 12, after kernel 4.4-rc1 and
>> QEMU 2.5-rc1, hoping things have calmed down a little bit.
>>
>> This RFC allows to set up AMD XGBE passthrough. This was tested on AMD
>> Seattle.
>>
>> The first upstreamed device supporting KVM platform passthrough was the
>> Calxeda Midway XGMAC. Compared to this latter, the XGBE XGMAC exposes a
>> much more complex device tree node. Generating the device tree node for
>> the guest is the challenging and controversary part of this series.
>>
>> - First There are 2 device tree node formats:
>> one where XGBE and PHY are described in separate nodes and another one
>> that combines both description in a single node (only supported by 4.2
>> onwards kernels). Only the combined description is supported for passthrough,
>> meaning the host must be >= 4.2 and must feature a device tree with a 
>> combined
>> description. The guest will also be exposed with a combined description,
>> meaning only >= 4.2 guest are supported. It is not planned to support
>> separate node representation since assignment of the PHY is less
>> straigtforward.
>>
>> - the XGMAC/PHY node depends on 2 clock nodes (DMA and PTP).
>> The code checks those clocks are fixed to make sure they cannot be
>> switched off at some point after the native driver gets unbound.
>>
>> - there are many property values to populate on guest side. Most of them
>> cannot be hardcoded. That series proposes a way to parse the host device
>> tree blob and retrieve host values to feed guest representation. Current
>> approach relies on dtc binary availability plus libfdt usage.
>> Other alternatives were discussed in:
>> http://www.spinics.net/lists/kvm-arm/msg16648.html.
>>
>> - Currently host booted with ACPI is not supported.
>
> I won't pretend to know all the politics in the ARM space, but doesn't
> this last bullet sort of imply that this is dead-on-arrival code?  Maybe
> not in the embedded space, but certainly in the server space, I thought
> ACPI was declared the winner.  Thanks,
>
Hi Alex,

We currently have no solution to accomplish platform device
passthrough on ACPI, because we don't know how to extract and
encapsulate the necessary information from the host about the
passthrough device and its dependency (clocks and phys for example).

There are people interested in this work, in particular in the
networking space, who couldn't care less about ACPI, and for those,
supporting platform device passthrough with DT remains highly
relevant.

Therefore, I think it's useful to consider these patches for review
and for their basic approach.

Thanks,
-Christoffer