Messages in this thread | | | Date | Wed, 19 Aug 2009 08:40:33 +0300 | From | Avi Kivity <> | Subject | Re: [Alacrityvm-devel] [PATCH v3 3/6] vbus: add a "vbus-proxy" bus model for vbus_driver objects |
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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.
>> (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).
>>> 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.
>>> 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.
>>> >>> >> 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.
> 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 have a truly marvellous patch that fixes the bug which this signature is too narrow to contain.
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