Subject: Re: [Alacrityvm-devel] [PATCH v3 3/6] vbus: add a "vbus-proxy" bus model for vbus_driver objects

From: "Ira W. Snyder" <[email-masked]>
Date: Wed, 19 Aug 2009 08:28:11 -0700

On Wed, Aug 19, 2009 at 08:40:33AM +0300, Avi Kivity wrote:
> On 08/19/2009 03:38 AM, Ira W. Snyder wrote:
>> On Wed, Aug 19, 2009 at 12:26:23AM +0300, Avi Kivity wrote:
>>    
>>> On 08/18/2009 11:59 PM, Ira W. Snyder wrote:
>>>      
>>>> On a non shared-memory system (where the guest's RAM is not just a chunk
>>>> of userspace RAM in the host system), virtio's management model seems to
>>>> fall apart. Feature negotiation doesn't work as one would expect.
>>>>
>>>>        
>>> In your case, virtio-net on the main board accesses PCI config space
>>> registers to perform the feature negotiation; software on your PCI cards
>>> needs to trap these config space accesses and respond to them according
>>> to virtio ABI.
>>>
>>>      
>> Is this "real PCI" (physical hardware) or "fake PCI" (software PCI
>> emulation) that you are describing?
>>
>>    
>
> Real PCI.
>
>> The host (x86, PCI master) must use "real PCI" to actually configure the
>> boards, enable bus mastering, etc. Just like any other PCI device, such
>> as a network card.
>>
>> On the guests (ppc, PCI agents) I cannot add/change PCI functions (the
>> last .[0-9] in the PCI address) nor can I change PCI BAR's once the
>> board has started. I'm pretty sure that would violate the PCI spec,
>> since the PCI master would need to re-scan the bus, and re-assign
>> addresses, which is a task for the BIOS.
>>    
>
> Yes.  Can the boards respond to PCI config space cycles coming from the  
> host, or is the config space implemented in silicon and immutable?   
> (reading on, I see the answer is no).  virtio-pci uses the PCI config  
> space to configure the hardware.
>

Yes, the PCI config space is implemented in silicon. I can change a few
things (mostly PCI BAR attributes), but not much.

>>> (There's no real guest on your setup, right?  just a kernel running on
>>> and x86 system and other kernels running on the PCI cards?)
>>>
>>>      
>> Yes, the x86 (PCI master) runs Linux (booted via PXELinux). The ppc's
>> (PCI agents) also run Linux (booted via U-Boot). They are independent
>> Linux systems, with a physical PCI interconnect.
>>
>> The x86 has CONFIG_PCI=y, however the ppc's have CONFIG_PCI=n. Linux's
>> PCI stack does bad things as a PCI agent. It always assumes it is a PCI
>> master.
>>
>> It is possible for me to enable CONFIG_PCI=y on the ppc's by removing
>> the PCI bus from their list of devices provided by OpenFirmware. They
>> can not access PCI via normal methods. PCI drivers cannot work on the
>> ppc's, because Linux assumes it is a PCI master.
>>
>> To the best of my knowledge, I cannot trap configuration space accesses
>> on the PCI agents. I haven't needed that for anything I've done thus
>> far.
>>
>>    
>
> Well, if you can't do that, you can't use virtio-pci on the host.   
> You'll need another virtio transport (equivalent to "fake pci" you  
> mentioned above).
>

Ok.

Is there something similar that I can study as an example? Should I look
at virtio-pci?

>>>> This does appear to be solved by vbus, though I haven't written a
>>>> vbus-over-PCI implementation, so I cannot be completely sure.
>>>>
>>>>        
>>> Even if virtio-pci doesn't work out for some reason (though it should),
>>> you can write your own virtio transport and implement its config space
>>> however you like.
>>>
>>>      
>> This is what I did with virtio-over-PCI. The way virtio-net negotiates
>> features makes this work non-intuitively.
>>    
>
> I think you tried to take two virtio-nets and make them talk together?   
> That won't work.  You need the code from qemu to talk to virtio-net  
> config space, and vhost-net to pump the rings.
>

It *is* possible to make two unmodified virtio-net's talk together. I've
done it, and it is exactly what the virtio-over-PCI patch does. Study it
and you'll see how I connected the rx/tx queues together.

The feature negotiation code also works, but in a very unintuitive
manner. I made it work in the virtio-over-PCI patch, but the devices are
hardcoded into the driver. It would be quite a bit of work to swap
virtio-net and virtio-console, for example.

>>>> I'm not at all clear on how to get feature negotiation to work on a
>>>> system like mine. From my study of lguest and kvm (see below) it looks
>>>> like userspace will need to be involved, via a miscdevice.
>>>>
>>>>        
>>> I don't see why.  Is the kernel on the PCI cards in full control of all
>>> accesses?
>>>
>>>      
>> I'm not sure what you mean by this. Could you be more specific? This is
>> a normal, unmodified vanilla Linux kernel running on the PCI agents.
>>    
>
> I meant, does board software implement the config space accesses issued  
> from the host, and it seems the answer is no.
>
>
>> In my virtio-over-PCI patch, I hooked two virtio-net's together. I wrote
>> an algorithm to pair the tx/rx queues together. Since virtio-net
>> pre-fills its rx queues with buffers, I was able to use the DMA engine
>> to copy from the tx queue into the pre-allocated memory in the rx queue.
>>
>>    
>
> Please find a name other than virtio-over-PCI since it conflicts with  
> virtio-pci.  You're tunnelling virtio config cycles (which are usually  
> done on pci config cycles) on a new protocol which is itself tunnelled  
> over PCI shared memory.
>

Sorry about that. Do you have suggestions for a better name?

I called it virtio-over-PCI in my previous postings to LKML, so until a
new patch is written and posted, I'll keep referring to it by the name
used in the past, so people can search for it.

When I post virtio patches, should I CC another mailing list in addition
to LKML?

>>>>
>>>>        
>>> Yeah.  You'll need to add byteswaps.
>>>
>>>      
>> I wonder if Rusty would accept a new feature:
>> VIRTIO_F_NET_LITTLE_ENDIAN, which would allow the virtio-net driver to
>> use LE for all of it's multi-byte fields.
>>
>> I don't think the transport should have to care about the endianness.
>>    
>
> Given this is not mainstream use, it would have to have zero impact when  
> configured out.
>

Yes, of course.

That said, I'm not sure how qemu-system-ppc running on x86 could
possibly communicate using virtio-net. This would mean the guest is an
emulated big-endian PPC, while the host is a little-endian x86. I
haven't actually tested this situation, so perhaps I am wrong.

>> True. It's slowpath setup, so I don't care how fast it is. For reasons
>> outside my control, the x86 (PCI master) is running a RHEL5 system. This
>> means glibc-2.5, which doesn't have eventfd support, AFAIK. I could try
>> and push for an upgrade. This obviously makes cat/echo really nice, it
>> doesn't depend on glibc, only the kernel version.
>>
>> I don't give much weight to the above, because I can use the eventfd
>> syscalls directly, without glibc support. It is just more painful.
>>    
>
> The x86 side only needs to run virtio-net, which is present in RHEL 5.3.  
> You'd only need to run virtio-tunnel or however it's called.  All the 
> eventfd magic takes place on the PCI agents.
>

I can upgrade the kernel to anything I want on both the x86 and ppc's.
I'd like to avoid changing the x86 (RHEL5) userspace, though. On the
ppc's, I have full control over the userspace environment.

Thanks,
Ira