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

[Christian Schoenebeck <qemu_oss@crudebyte.com>]

On Dienstag, 23. Februar 2021 15:07:31 CET Michael S. Tsirkin wrote:
> > 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.

Misapprehension + typo(s) in my previous message, sorry Michael. That's 500k 
of course (not 5k), yes.

Let me rephrase that question: are you aware of something in virtio that would 
per se mandate an absolute hard coded message size limit (e.g. from virtio 
specs perspective or maybe some compatibility issue)?

If not, we would try getting rid of that hard coded limit of the 9p client on 
kernel side in the first place, because the kernel's 9p client already has a 
dynamic runtime option 'msize' and that hard coded enforced limit (500k) is a 
performance bottleneck like I said.

Best regards,
Christian Schoenebeck