Re: [RFC v3 3/3] vhost: Allocate memory for packed vring

[Sahil <icegambit91@gmail.com>]

Hi,

On Tuesday, August 27, 2024 9:00:36 PM GMT+5:30 Eugenio Perez Martin wrote:
> On Wed, Aug 21, 2024 at 2:20 PM Sahil <icegambit91@gmail.com> wrote:
> > [...]
> > I have been trying to test my changes so far as well. I am not very clear
> > on a few things.
> > 
> > Q1.
> > I built QEMU from source with my changes and followed the vdpa_sim +
> > vhost_vdpa tutorial [1]. The VM seems to be running fine. How do I check
> > if the packed format is being used instead of the split vq format for
> > shadow virtqueues? I know the packed format is used when virtio_vdev has
> > got the VIRTIO_F_RING_PACKED bit enabled. Is there a way of checking that
> > this is the case?
> 
> You can see the features that the driver acked from the guest by
> checking sysfs. Once you know the PCI BFN from lspci:
> # lspci -nn|grep '\[1af4:1041\]'
> 01:00.0 Ethernet controller [0200]: Red Hat, Inc. Virtio 1.0 network
> device [1af4:1041] (rev 01)
> # cut -c 35
> /sys/devices/pci0000:00/0000:00:02.0/0000:01:00.0/virtio0/features 0
> 
> Also, you can check from QEMU by simply tracing if your functions are
> being called.
> 
> > Q2.
> > What's the recommended way to see what's going on under the hood? I tried
> > using the -D option so QEMU's logs are written to a file but the file was
> > empty. Would using qemu with -monitor stdio or attaching gdb to the QEMU
> > VM be worthwhile?
> 
> You need to add --trace options with the regex you want to get to
> enable any output. For example, --trace 'vhost_vdpa_*' print all the
> trace_vhost_vdpa_* functions.
> 
> If you want to speed things up, you can just replace the interesting
> trace_... functions with fprintf(stderr, ...). We can add the trace
> ones afterwards.

Understood. I am able to trace the functions that are being called with
fprintf. I'll stick with fprintf for now.

I realized that packed vqs are not being used in the test environment. I
see that in "hw/virtio/vhost-shadow-virtqueue.c", svq->is_packed is set
to 0 and that calls vhost_svq_add_split(). I am not sure how one enables
the packed feature bit. I don't know if this is an environment issue.

I built qemu from the latest source with my changes on top of it. I followed
this article [1] to set up the environment.

On the host machine:

$ uname -a
Linux fedora 6.10.5-100.fc39.x86_64 #1 SMP PREEMPT_DYNAMIC Wed Aug 14 15:49:25 UTC 2024 x86_64 GNU/Linux

$ ./qemu/build/qemu-system-x86_64 --version
QEMU emulator version 9.0.91

$ vdpa -V
vdpa utility, iproute2-6.4.0

All the relevant vdpa modules have been loaded in accordance with [1].

$ lsmod | grep -iE "(vdpa|virtio)"
vdpa_sim_net	12288  0
vdpa_sim		24576  1 vdpa_sim_net
vringh		32768  2 vdpa_sim,vdpa_sim_net
vhost_vdpa		32768  2
vhost		65536  1 vhost_vdpa
vhost_iotlb		16384  4 vdpa_sim,vringh,vhost_vdpa,vhost
vdpa		36864  3 vdpa_sim,vhost_vdpa,vdpa_sim_net

$ ls -l /sys/bus/vdpa/devices/vdpa0/driver
lrwxrwxrwx. 1 root root 0 Aug 30 11:25 /sys/bus/vdpa/devices/vdpa0/driver -> ../../bus/vdpa/drivers/vhost_vdpa

In the output of the following command, I see ANY_LAYOUT is supported.
According to virtio_config.h [2] in the linux kernel, this represents the
layout of descriptors. This refers to split and packed vqs, right?

$ vdpa mgmtdev show
vdpasim_net: 
  supported_classes net
  max_supported_vqs 3
  dev_features MTU MAC STATUS CTRL_VQ CTRL_MAC_ADDR ANY_LAYOUT VERSION_1 ACCESS_PLATFORM

$ vdpa dev show -jp
{
    "dev": {
        "vdpa0": {
            "type": "network",
            "mgmtdev": "vdpasim_net",
            "vendor_id": 0,
            "max_vqs": 3,
            "max_vq_size": 256
        }
    }
}

I started the VM by running:

$ sudo ./qemu/build/qemu-system-x86_64 \
-enable-kvm \
-drive file=//home/ig91/fedora_qemu_test_vm/L1.qcow2,media=disk,if=virtio \
-net nic,model=virtio \
-net user,hostfwd=tcp::2226-:22 \
-netdev type=vhost-vdpa,vhostdev=/dev/vhost-vdpa-0,id=vhost-vdpa0 \
-device virtio-net-pci,netdev=vhost-vdpa0,bus=pci.0,addr=0x7,disable-legacy=on,disable-modern=off,page-per-vq=on,event_idx=off,packed=on \
-nographic \
-m 2G \
-smp 2 \
-cpu host \
2>&1 | tee vm.log

I added the packed=on option to -device virtio-net-pci.

In the VM:

# uname -a
Linux fedora 6.8.5-201.fc39.x86_64 #1 SMP PREEMPT_DYNAMIC Thu Apr 11 18:25:26 UTC 2024 x86_64 GNU/Linux

# lspci -nn | grep -i -A15 "\[1af4:1041\]"
00:07.0 Ethernet controller [0200]: Red Hat, Inc. Virtio 1.0 network device [1af4:1041] (rev 01)

# cut -c 35 /sys/devices/pci0000:00/0000:00:07.0/virtio1/features
0

The packed vq feature bit hasn't been set. Am I missing something here?

Thanks,
Sahil

[1] https://www.redhat.com/en/blog/hands-vdpa-what-do-you-do-when-you-aint-got-hardware-part-1
[2] https://github.com/torvalds/linux/blob/master/include/uapi/linux/virtio_config.h#L63