Re: [virtio-dev] Re: [PATCH v9 02/10] admin: introduce device group and related concepts

[Jason Wang <jasowang@redhat.com>]

在 2022/11/24 20:12, Cornelia Huck 写道:
> On Thu, Nov 24 2022, "Michael S. Tsirkin" <mst@redhat.com> wrote:
>
>> On Thu, Nov 24, 2022 at 03:37:42PM +0800, Jason Wang wrote:
>>> On Thu, Nov 24, 2022 at 3:08 PM Michael S. Tsirkin <mst@redhat.com> wrote:
>>>> On Thu, Nov 24, 2022 at 01:41:41PM +0800, Jason Wang wrote:
>>>>> On Thu, Nov 24, 2022 at 5:08 AM Michael S. Tsirkin <mst@redhat.com> wrote:
>>>>>> +The following group types, and their identifiers, are currently specified):
>>>>>> +\begin{description}
>>>>>> +\item[SR-IOV group type (0x1)]
>>>>>> +This device group has a PCI Single Root I/O Virtualization
>>>>>> +(SR-IOV) physical function (PF) device as the owner and includes
>>>>>> +all its SR-IOV virtual functions (VFs) as members (see
>>>>>> +\hyperref[intro:PCIe]{[PCIe]}).
>>>>> So I wonder what's the advantage of using a global identifier over the
>>>>> transport specific one. There's almost no way for CCW/MMIO to use
>>>>> SR-IOV. Limiting it to PCI seems much easier and avoids layer
>>>>> violation.
>>>>>
>>>>> Thanks
>>>> So we burn up an identifier, ccw and mmio won't be able to use it.
>>>> Big deal? Why?
>>> Because it is transport specific. The basic facility should be
>>> transport independent.
>> I tried this but the result is spread all over the spec
>> and does not result in a readable cohesive whole.


I may miss something, but it looks to me it's just a subsection in the 
PCI transport to describe the SR-IOV group identifier.


>> So we give up on the transport independent purity a bit and it
>> seems worth it.
>> Also explained this in the cover letter - have you seen that?


Sorry, I don't.


>>
>>
>>
>>>> And I think we might find a use for this with MMIO
>>>> down the road with some kind of passthrough. Who knows.
>>> Probably, but can it be modeled exactly as what SR-IOV looks like? Or
>>> anyhow, it's not too late to define this for MMIO at that time. For
>>> example, we know MSI-X may work for MMIO but we define it only for PCI
>>> now.
>>>
>>> Thanks
>> Right. So if we reserve the id for all transports then it will
>> be easy to do.
> Also, if we go with transport-specific ids, we might end up with
> different ids per transport when we add a future transport-independent
> feature.


I'm not sure this is a real problem:

1) all the basic facilities are now transport-independent but they are 
implemented via different transport specific registers/commands/offsets etc.
2) I'm not quite sure there would be a transport-independent group 
identifier other than the virtqueue transport, so I wonder if it makes 
sense to split the id space into global ones as well as the transport 
specific ones


>   The global id space really should be big enough to accommodate
> even single-transport groups.


Yes, so it's not about the space but about whether or it it's 
worth/correct/expensive to describe a transport specific identifier in 
the basic facility part.

Thanks


>
>
> ---------------------------------------------------------------------
> To unsubscribe, e-mail: virtio-dev-unsubscribe@lists.oasis-open.org
> For additional commands, e-mail: virtio-dev-help@lists.oasis-open.org
>