Re: [RFC v4 0/5] Add packed virtqueue to shadow virtqueue

[Sahil Siddiq <icegambit91@gmail.com>]

Hi,

Thank you for your reply.

On 12/10/24 2:57 PM, Eugenio Perez Martin wrote:
> On Thu, Dec 5, 2024 at 9:34 PM Sahil Siddiq <icegambit91@gmail.com> wrote:
>>
>> Hi,
>>
>> There are two issues that I found while trying to test
>> my changes. I thought I would send the patch series
>> as well in case that helps in troubleshooting. I haven't
>> been able to find an issue in the implementation yet.
>> Maybe I am missing something.
>>
>> I have been following the "Hands on vDPA: what do you do
>> when you ain't got the hardware v2 (Part 2)" [1] blog to
>> test my changes. To boot the L1 VM, I ran:
>>
>> [...]
>>
>> But if I boot L2 with x-svq=true as shown below, I am unable
>> to ping the host machine.
>>
>> $ ./qemu/build/qemu-system-x86_64 \
>> -nographic \
>> -m 4G \
>> -enable-kvm \
>> -M q35 \
>> -drive file=//root/L2.qcow2,media=disk,if=virtio \
>> -netdev type=vhost-vdpa,vhostdev=/dev/vhost-vdpa-0,x-svq=true,id=vhost-vdpa0 \
>> -device virtio-net-pci,netdev=vhost-vdpa0,disable-legacy=on,disable-modern=off,ctrl_vq=on,ctrl_rx=on,event_idx=off,bus=pcie.0,addr=0x7 \
>> -smp 4 \
>> -cpu host \
>> 2>&1 | tee vm.log
>>
>> In L2:
>>
>> # ip addr add 111.1.1.2/24 dev eth0
>> # ip addr show eth0
>> 2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
>>      link/ether 52:54:00:12:34:57 brd ff:ff:ff:ff:ff:ff
>>      altname enp0s7
>>      inet 111.1.1.2/24 scope global eth0
>>         valid_lft forever preferred_lft forever
>>      inet6 fe80::9877:de30:5f17:35f9/64 scope link noprefixroute
>>         valid_lft forever preferred_lft forever
>>
>> # ip route
>> 111.1.1.0/24 dev eth0 proto kernel scope link src 111.1.1.2
>>
>> # ping 111.1.1.1 -w10
>> PING 111.1.1.1 (111.1.1.1) 56(84) bytes of data.
>>  From 111.1.1.2 icmp_seq=1 Destination Host Unreachable
>> ping: sendmsg: No route to host
>>  From 111.1.1.2 icmp_seq=2 Destination Host Unreachable
>>  From 111.1.1.2 icmp_seq=3 Destination Host Unreachable
>>
>> --- 111.1.1.1 ping statistics ---
>> 3 packets transmitted, 0 received, +3 errors, 100% packet loss, time 2076ms
>> pipe 3
>>
>> The other issue is related to booting L2 with "x-svq=true"
>> and "packed=on".
>>
>> In L1:
>>
>> $ ./qemu/build/qemu-system-x86_64 \
>> -nographic \
>> -m 4G \
>> -enable-kvm \
>> -M q35 \
>> -drive file=//root/L2.qcow2,media=disk,if=virtio \
>> -netdev type=vhost-vdpa,vhostdev=/dev/vhost-vdpa-0,id=vhost-vdpa0,x-svq=true \
>> -device virtio-net-pci,netdev=vhost-vdpa0,disable-legacy=on,disable-modern=off,guest_uso4=off,guest_uso6=off,host_uso=off,guest_announce=off,ctrl_vq=on,ctrl_rx=on,event_idx=off,packed=on,bus=pcie.0,addr=0x7 \
>> -smp 4 \
>> -cpu host \
>> 2>&1 | tee vm.log
>>
>> The kernel throws "virtio_net virtio1: output.0:id 0 is not
>> a head!" [4].
>>
> 
> So this series implements the descriptor forwarding from the guest to
> the device in packed vq. We also need to forward the descriptors from
> the device to the guest. The device writes them in the SVQ ring.
> 
> The functions responsible for that in QEMU are
> hw/virtio/vhost-shadow-virtqueue.c:vhost_svq_flush, which is called by
> the device when used descriptors are written to the SVQ, which calls
> hw/virtio/vhost-shadow-virtqueue.c:vhost_svq_get_buf. We need to do
> modifications similar to vhost_svq_add: Make them conditional if we're
> in split or packed vq, and "copy" the code from Linux's
> drivers/virtio/virtio_ring.c:virtqueue_get_buf.
> 
> After these modifications you should be able to ping and forward
> traffic. As always, It is totally ok if it needs more than one
> iteration, and feel free to ask any question you have :).
> 

Understood, I'll make these changes and will test it again.

Thanks,
Sahil