Re: [PATCH v6] can: virtio: Add virtio CAN driver

[Francesco Valla <francesco@valla.it>]

Hi Matias,

On Wed, Jan 07, 2026 at 05:14:25PM +0100, Matias Ezequiel Vara Larsen wrote:
> On Fri, Dec 26, 2025 at 4:09 PM Francesco Valla <francesco@valla.it> wrote:
> >
> > Hi,
> >
> > On Sun, Dec 14, 2025 at 04:25:54PM +0100, Francesco Valla wrote:
> > > While stress testing this, I noticed that flooding the virtio-can
> > > interface with packets leads to an hang of the interface itself.
> > > I am seeing this issuing, at host side:
> > >
> > >       while true; do cansend can0 123#00; done
> > >
> > > with:
> > >
> > >  - QEMU: the tip of the master branch plus [2]
> > >  - vhost-device: the tip of the main branch
> > >
> > > and the following QEMU invocation:
> > >
> > > qemu-system-x86_64 -serial mon:stdio \
> > >     -m 2G -smp 2 \
> > >     -kernel $(pwd)/BUILD.bin/arch/x86/boot/bzImage \
> > >     -initrd /home/francesco/SRC/LINUX_KERNEL/initramfs.gz \
> > >     -append "loglevel=7 console=ttyS0" \
> > >     -machine memory-backend=pc.ram \
> > >     -object memory-backend-file,id=pc.ram,size=2G,mem-path=/tmp/pc.ram,share=on \
> > >     -chardev socket,id=can0,path=/tmp/sock-can0 \
> > >     -device vhost-user-can-pci,chardev=can0
> > >
> > >
> > > Restarting the interface (i.e.: ip link set down and the up) does not
> > > fix the situation.
> > >
> > > I'll try to do some more testing during the next days.
> >
> > After a deep dive, I _think_ the problem actually lies in vhost-device,
> > since it is not there (or al least, it seems so) using an alternative
> > implementation that uses the qemu socketcan support [0] (implementation
> > which builds on top of the work done by Harald and Mikhail):
> >
> > qemu-system-x86_64 -serial mon:stdio \
> >     -m 2G -smp 2 -enable-kvm \
> >     -kernel $(pwd)/BUILD.bin/arch/x86/boot/bzImage \
> >     -initrd /home/francesco/SRC/LINUX_KERNEL/initramfs.gz \
> >     -append "loglevel=7 console=ttyS0" \
> >     -object can-bus,id=canbus0 -object can-host-socketcan,id=canhost0,if=vcan0,canbus=canbus0 \
> >     -device virtio-can-pci,canbus=canbus0
> >
> > Unfortunately, my Rust knoweledge is not sufficient to understand the
> > vhost-device implementation [1]; the issue seems to be related to the
> > host->guest vring becoming empty and not refilling anymore.
> >
> 
> Can you try with
> https://github.com/MatiasVara/vhost-device/commits/fix-for-923/?

I'll stress test it during the night, but this seems to fix it. Before
it was reproducible in a consistent manner after mere seconds, while i
now in a bunch of runs I never reproduced it.

I also agree with your analysis on the commit.

> 
> Thanks, Matias
> 
> 

Thank you!

Francesco