Re: Attaching 9P VirtFS to PowerNV

[Greg Kurz]

On Wed, 3 Mar 2021 11:19:37 -0800
Ryan Burns <ryanxcv@gmail.com> wrote:

> Hi Murilo, thanks for your response.
> 

Hi Ryan,

It is probably best to Cc QEMU 9p maintainers and qemu-devel@nongnu.org
so that people who can provide help have a chance to see your question.

Luckily, I happen to spend most of my time on PPC :)

> On 3/3/21 8:37 AM, Murilo Opsfelder Araújo wrote:
> > Hi, Ryan.
> > 
> > On Wednesday, March 3, 2021 4:59:09 AM -03 Ryan Burns wrote:
> >> Hi all,
> >>
> >> I'm trying to port NixOS to run on QEMU's PowerNV platform. The system
> >> is able to load the kernel, initrd, and a virtio-blk-pci device without

Very cool ! Cc'ing PowerNV maintainer Cédric Le Goater. He'll probably be
interested in your initiative.

> >> issues, but fails trying to mount the nix store from the host via -virtfs.
> >>
> >> I noticed that unlike on other platforms, I needed to specify the PCIe
> >> bus for the virtio-blk-pci device to appear as /dev/vda. Do I need to
> >> do something similar for the 9P virtfs?

Probably. The -virtfs option is sugar for -fsdev + -device virtio-9p-pci.

You can check the QEMU manual page or the wiki to see how to convert
your command line:

https://wiki.qemu.org/Documentation/9psetup#Starting_the_Guest_directly

See below for further explanation.

> >>
> >>
> >> Here is the relevant snippet from my boot log:
> >>
> >>
> >> /dev/vda: recovering journal
> >> /dev/vda: clean, 231/32768 files, 10160/131072 blocks
> >> mounting /dev/vda on /...
> >> mounting store on /nix/.ro-store...
> >> [    5.972587][  T170] 9pnet_virtio: no channels available for device store
> >> [    5.972587][  T170] 9pnet_virtio: no channels available for device store

This message means that 9pnet_virtio doesn't know about the mount
tags that you have passed to -virtfs, i.e. very likely because the
virtio-9p-pci device isn't seen by the guest.

> >> mount: mounting store on /mnt-root/nix/.ro-store failed: No such file or
> >> directory
> > 
> > The message suggests that /mnt-root/nix/.ro-store does not exist at the 
> > moment
> > mount is called.
> > 
> > Are you 100% sure that the previous mkdir was successful?  Is "/" writable 
> > at
> > the moment mkdir was called?  Shouldn't the script check for the return 
> > code 
> > of
> > mkdir?
> > 
> >     https://github.com/NixOS/nixpkgs/blob/
> > 545bafa2d49d5184714d94ea4c568316d9ce7b5c/nixos/modules/system/boot/stage-1-
> > init.sh#L390
> >     mkdir -p "/mnt-root$mountPoint"
> 
> The previous mkdir was successful - "/" is always a fresh qcow2 image.
> I added some debugging commands to the stage 1 script to be sure -
> the mkdir command exits with status 0 and the mountpoint does exist.
> 
> > 
> > Besides that, I'd also check if filesystem type is missing from mount 
> > arguments,
> > as per:
> > 
> >     https://wiki.qemu.org/Documentation/9psetup#Mounting_the_shared_path
> 
> The stage 1 script does this by generating an fstab on-the-fly with the
> correct options set. Here is what it looks like at mount time:
> 
> /dev/vda /mnt-root/ ext4 defaults,
> store /mnt-root/nix/.ro-store 9p trans=virtio,version=9p2000.L,cache=loose,
> 
> I think this is all correct, right? I might add that these VM scripts are
> pretty well-tested on x86 and ARM, which is why I thought I'd ask on this
> mailing list - I assume the most likely explanation is that I'm simply
> using a faulty QEMU command-line or kernel config.
> 

The command line to start a PowerNV machine with multiple PCIe
devices requires to specify the bus for all of them. 

This is what I do to boot a fedora guest with a virtio-scsi,
a virtio-9p and a virtio-net device:

qemu-system-ppc64 \
        -machine powernv9 \
        -m 2G \
        -kernel ${kernel} -initrd ${initrd} -bios ${skiboot_fw} \
        -serial mon:stdio \
        -nographic -nodefaults \
        -drive file=${fedora_image},if=none,id=drive0 \
        -device virtio-scsi-pci,bus=pcie.0 \
        -device scsi-hd,drive=drive0,bootindex=1 \
        -netdev tap,helper=/usr/libexec/qemu-bridge-helper,br=virbr0,id=netdev0 
\
        -device virtio-net-pci,netdev=netdev0,mac=${mac},bus=pcie.1 \
        -fsdev local,id=fsdev0,path=${path},security_model=none \
        -device virtio-9p-pci,fsdev=fsdev0,mount_tag=host,bus=pcie.2

Cheers,

--
Greg

> > 
> > Cheers!
> > 
> >> And here is what my generated qemu command line invocation looks like:
> >>
> >>
> >> exec
> >> /nix/store/r2jn54cpvmp2620f6ky8yy2dfd3lglwz-qemu-5.2.0/bin/qemu-system-ppc64
> >> -machine powernv \
> >>     -name nixos \
> >>     -m 2048 \
> >>     -smp 1 \
> >>     -device virtio-rng-pci \
> >>     -net nic,netdev=user.0,model=virtio -netdev
> >> user,id=user.0${QEMU_NET_OPTS:+,$QEMU_NET_OPTS} \
> >>     -virtfs local,path=/nix/store,security_model=none,mount_tag=store \
> >>     -virtfs local,path=$TMPDIR/xchg,security_model=none,mount_tag=xchg \
> >>     -virtfs
> >> local,path=${SHARED_DIR:-$TMPDIR/xchg},security_model=none,mount_tag=shared
> >> \
> >>     -drive
> >> cache=writeback,file=$NIX_DISK_IMAGE,id=drive1,if=none,index=1,werror=report
> >> -device virtio-blk-pci,bus=pcie.0,drive=drive1 \
> >>     -kernel
> >> /nix/store/4mw3zzh1k4xhlysfmcgvpgx9zjz032c7-nixos-system-nixos-21.03pre-git/
> >> kernel -initrd
> >> /nix/store/4mw3zzh1k4xhlysfmcgvpgx9zjz032c7-nixos-system-nixos-21.03pre-git/
> >> initrd -append "$(cat
> >> /nix/store/4mw3zzh1k4xhlysfmcgvpgx9zjz032c7-nixos-system-nixos-21.03pre-git/
> >> kernel-params)
> >> init=/nix/store/4mw3zzh1k4xhlysfmcgvpgx9zjz032c7-nixos-system-nixos-21.03pr
> >> e-git/init
> >> regInfo=/nix/store/dnrqa7qar8jakalrz85qn3wslq5k7fmm-closure-info-powerpc64l
> >> e-unknown-linux-gnu/registration console=ttyAMA0,115200n8
> >> $QEMU_KERNEL_PARAMS" -nographic \
> > 
> > --
> > Murilo
> > 
>