RE: question about virtio-vsock on xen

[Peng Fan <peng.fan@xxxxxxx>]

Hi Stefano,

> Subject: Re: question about virtio-vsock on xen
> 
> Hi Peng,
> 
> We haven't tried to setup virtio-vsock yet.
> 
> In general, I am very supportive of using QEMU for virtio backends. We use
> QEMU to provide virtio-net, virtio-block, virtio-console and more.

Would you mind share how to setup virtio-console using qemu + xen?

> 
> However, typically virtio-vsock comes into play for VM-to-VM
> communication, which is different. Going via QEMU in Dom0 just to have 1
> VM communicate with another VM is not an ideal design: it adds latency and
> uses resources in Dom0 when actually we could do without it.
> 
> A better model for VM-to-VM communication would be to have the VM talk
> to each other directly via grant table or pre-shared memory (see the static
> shared memory feature) or via Xen hypercalls (see Argo.)

The goal is to make android trout VM run with XEN + i.MX95, so need vsock.

Thanks,
Peng.

> 
> For a good Xen design, I think the virtio-vsock backend would need to be in
> Xen itself (the hypervisor).
> 
> Of course that is more work and it doesn't help you with the specific question
> you had below :-)
> 
> For that, I don't have a pointer to help you but maybe others in CC have.
> 
> Cheers,
> 
> Stefano
> 
> 
> On Fri, 23 Feb 2024, Peng Fan wrote:
> > Hi All,
> >
> > Has anyone make virtio-vsock on xen work? My dm args as below:
> >
> > virtio = [
> >
> 'backend=0,type=virtio,device,transport=pci,bdf=05:00.0,backend_type=qem
> u,grant_usage=true'
> > ]
> > device_model_args = [
> > '-D', '/home/root/qemu_log.txt',
> > '-d',
> > 'trace:*vsock*,trace:*vhost*,trace:*virtio*,trace:*pci_update*,trace:*
> > pci_route*,trace:*handle_ioreq*,trace:*xen*',
> > '-device',
> > 'vhost-vsock-pci,iommu_platform=false,id=vhost-vsock-pci0,bus=pcie.0,a
> > ddr=5.0,guest-cid=3']
> >
> > During my test, it always return failure in dom0 kernel in below code:
> >
> > vhost_transport_do_send_pkt {
> > ...
> >                nbytes = copy_to_iter(hdr, sizeof(*hdr), &iov_iter);
> >                 if (nbytes != sizeof(*hdr)) {
> >                         vq_err(vq, "Faulted on copying pkt hdr %x %x %x 
> > %px\n",
> nbytes, sizeof(*hdr),
> > __builtin_object_size(hdr, 0), &iov_iter);
> >                         kfree_skb(skb);
> >                         break;
> >                 }
> > }
> >
> > I checked copy_to_iter, it is copy data to __user addr, but it never
> > pass, the copy to __user addr always return 0 bytes copied.
> >
> > The asm code "sttr x7, [x6]" will trigger data abort, the kernel will
> > run into do_page_fault, but lock_mm_and_find_vma report it is
> > VM_FAULT_BADMAP, that means the __user addr is not mapped, no vma
> has this addr.
> >
> > I am not sure what may cause this. Appreciate if any comments.
> >
> > BTW: I tested blk pci, it works, so the virtio pci should work on my setup.
> >
> > Thanks,
> > Peng.
> >