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

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Feb 23, 2021 at 02:39:48PM +0100, Christian Schoenebeck wrote:
> 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>

Well the change I see is:

-       .maxsize = PAGE_SIZE*VIRTQUEUE_NUM,
+       .maxsize = PAGE_SIZE * (VIRTQUEUE_NUM - 3),


so how come you say it changes 512K to 5K?
Looks more like 500K to me.

> 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
>