Re: Intention to work on GSoC project

[Sahil]

Hi,

Thank you for your reply.

On Monday, April 15, 2024 2:27:36 PM IST Eugenio Perez Martin wrote:
> [...]
> > I have one question though. One of the options (use case 1 in [1])
> > 
> > given to the "qemu-kvm" command is:
> > > -device virtio-net-pci,netdev=vhost-vdpa0,bus=pcie.0,addr=0x7\
> > > ,disable-modern=off,page-per-vq=on
> > 
> > This gives an error:
> > > Bus "pcie.0" not found
> > 
> > Does pcie refer to PCI Express? Changing this to pci.0 works.
> 
> Yes, you don't need to mess with pcie stuff so this solution is
> totally valid. I think we need to change that part in the tutorial.
> 

Understood.

> > I read through the "device buses" section in QEMU's user
> > documentation [5], but I have still not understood this.
> > 
> > "ls /sys/bus/pci/devices/* | grep vdpa" does not give any results.
> > Replacing pci with pci_express doesn't give any results either. How
> > does one know which pci bus the vdpa device is connected to?
> > I have gone through the "vDPA bus drivers" section of the "vDPA
> > kernel framework" article [6] but I haven't managed to find an
> > answer yet. Am I missing something here?
> 
> You cannot see the vDPA device from the guest. From the guest POV is a
> regular virtio over PCI bus.
>
> From the host, vdpa_sim is not a PCI device either, so you cannot see
> under /sys/bus. Do you have a vdpa* entry under
> /sys/bus/vdpa/devices/?
>

After re-reading the linked articles, I think I have got some more
clarity. One confusion was related to the difference between vdpa
and vhost-vdpa.

So far what I have understood is that L0 acts as the host and L1
acts as the guest in this setup. I understand that the guest can't
see the vDPA device.

I now also understand that vdpa_sim is not a PCI device. I am also
under the impression that vdpa refers to the vdpa bus while
vhost-vdpa is the device. Is my understanding correct?

After running the commands in the blog [1], I see that there's a
vhost-vdpa-0 device under /dev.

I also have an entry "vdpa0" under /sys/bus/vdpa/devices/ which
is a symlink to /sys/devices/vdpa0. There's a dir "vhost-vdpa-0"
under "/sys/devices/vdpa0". Hypothetically, if vhost-vdpa-0 had
been a PCI device, then it would have been present under
/sys/bus/pci/devices, right?

Another source of confusion was the pci.0 option passed to the
qemu-kvm command. But I have understood this as well now:
"-device virtio-net-pci" is a pci device.
 
> > There's one more thing. In "use case 1" of "Running traffic with
> > vhost_vdpa in Guest" [1], running "modprobe pktgen" in the L1 VM
> > 
> > gives an error:
> > > module pktgen couldn't be found in /lib/modules/6.5.6-300.fc39.x86_64.
> > 
> > The kernel version is 6.5.6-300.fc39.x86_64. I haven't tried building
> > pktgen manually in L1. I'll try that and will check if vdpa_sim works
> > as expected after that.
> 
> Did you install kernel-modules-internal?

I just realized I had the wrong version of kernel-modules-internal
installed. It works after installing the right version.

Thanks,
Sahil

[1] 
https://www.redhat.com/en/blog/hands-vdpa-what-do-you-do-when-you-aint-got-hardware-part-1