Re: Can not set high msize with virtio-9p (Was: Re: virtiofs vs 9p performance)

[Christian Schoenebeck <qemu_oss@crudebyte.com>]

On Montag, 22. Februar 2021 18:11:59 CET Greg Kurz wrote:
> On Mon, 22 Feb 2021 16:08:04 +0100
> Christian Schoenebeck <qemu_oss@crudebyte.com> wrote:
> 
> [...]
> 
> > I did not ever have a kernel crash when I boot a Linux guest with a 9pfs
> > root fs and 100 MiB msize.
> 
> Interesting.
> 
> > Should we ask virtio or 9p Linux client maintainers if
> > they can add some info what this is about?
> 
> Probably worth to try that first, even if I'm not sure anyone has a
> answer for that since all the people who worked on virtio-9p at
> the time have somehow deserted the project.

Michael, Dominique,

we are wondering here about the message size limitation of just 5 kiB in the 
9p Linux client (using virtio transport) which imposes a performance 
bottleneck, introduced by this kernel commit:

commit b49d8b5d7007a673796f3f99688b46931293873e
Author: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Date:   Wed Aug 17 16:56:04 2011 +0000

    net/9p: Fix kernel crash with msize 512K
    
    With msize equal to 512K (PAGE_SIZE * VIRTQUEUE_NUM), we hit multiple
    crashes. This patch fix those.
    
    Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
    Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>

Is this a fundamental maximum message size that cannot be exceeded with virtio 
in general or is there another reason for this limit that still applies?

Full discussion:
https://lists.gnu.org/archive/html/qemu-devel/2021-02/msg06343.html

> > > > As the kernel code sais trans_mod->maxsize, maybe its something in
> > > > virtio
> > > > on qemu side that does an automatic step back for some reason. I don't
> > > > see something in the 9pfs virtio transport driver
> > > > (hw/9pfs/virtio-9p-device.c on QEMU side) that would do this, so I
> > > > would
> > > > also need to dig deeper.
> > > > 
> > > > Do you have some RAM limitation in your setup somewhere?
> > > > 
> > > > For comparison, this is how I started the VM:
> > > > 
> > > > ~/git/qemu/build/qemu-system-x86_64 \
> > > > -machine pc,accel=kvm,usb=off,dump-guest-core=off -m 2048 \
> > > > -smp 4,sockets=4,cores=1,threads=1 -rtc base=utc \
> > > > -boot strict=on -kernel
> > > > /home/bee/vm/stretch/boot/vmlinuz-4.9.0-13-amd64 \
> > > > -initrd /home/bee/vm/stretch/boot/initrd.img-4.9.0-13-amd64 \
> > > > -append 'root=svnRoot rw rootfstype=9p
> > > > rootflags=trans=virtio,version=9p2000.L,msize=104857600,cache=mmap
> > > > console=ttyS0' \
> > > 
> > > First obvious difference I see between your setup and mine is that
> > > you're mounting the 9pfs as root from the kernel command line. For
> > > some reason, maybe this has an impact on the check in p9_client_create()
> > > ?
> > > 
> > > Can you reproduce with a scenario like Vivek's one ?
> > 
> > Yep, confirmed. If I boot a guest from an image file first and then try to
> > manually mount a 9pfs share after guest booted, then I get indeed that
> > msize capping of just 512 kiB as well. That's far too small. :/
> 
> Maybe worth digging :
> - why no capping happens in your scenario ?

Because I was wrong.

I just figured even in the 9p rootfs scenario it does indeed cap msize to 5kiB 
as well. The output of /etc/mtab on guest side was fooling me. I debugged this 
on 9p server side and the Linux 9p client always connects with a max. msize of 
5 kiB, no matter what you do.

> - is capping really needed ?
> 
> Cheers,

That's a good question and probably depends on whether there is a limitation 
on virtio side, which I don't have an answer for. Maybe Michael or Dominique 
can answer this.

Best regards,
Christian Schoenebeck