Re: virtio 4M limit

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

On Sun, Oct 03, 2021 at 08:14:55PM +0200, Christian Schoenebeck wrote:
> On Freitag, 1. Oktober 2021 13:21:23 CEST Christian Schoenebeck wrote:
> > Hi Michael,
> > 
> > while testing the following kernel patches I realized there is currently a
> > size limitation of 4 MB with virtio on QEMU side:
> > https://lore.kernel.org/netdev/cover.1632327421.git.linux_oss@crudebyte.com/
> > 
> > So with those kernel patches applied I can mount 9pfs on Linux guest with
> > the 9p 'msize' (maximum message size) option with a value of up to 4186112
> > successfully. If I try to go higher with 'msize' then the system would hang
> > with the following QEMU error:
> > 
> >   qemu-system-x86_64: virtio: too many write descriptors in indirect table
> > 
> > Which apparently is due to the amount of scatter gather lists on QEMU virtio
> > side currently being hard coded to 1024 (i.e. multiplied by 4k page size =>
> > 4 MB):
> > 
> >   ./include/hw/virtio/virtio.h:
> >   #define VIRTQUEUE_MAX_SIZE 1024
> > 
> > Is that hard coded limit carved into stone for some reason or would it be OK
> > if I change that into a runtime variable?
> 
> After reviewing the code and protocol specs, it seems that this value is
> simply too small. I will therefore send a patch suggsting to raise this value
> to 32768, as this is the maximum possible value according to the virtio specs.
> 
> https://docs.oasis-open.org/virtio/virtio/v1.1/cs01/virtio-v1.1-cs01.html#x1-240006

I think it's too aggressive to change it for all devices.
Pls find a way to only have it affect 9pfs.

> > If that would be Ok, maybe something similar that I did with those kernel
> > patches, i.e. retaining 1024 as an initial default value and if indicated
> > from guest side that more is needed, increasing the SG list amount
> > subsequently according to whatever is needed by guest?
> 
> Further changes are probably not necessary.
> 
> The only thing I have spotted that probably should be changed is that at some
> few locations, a local array is allocated on the stack with VIRTQUEUE_MAX_SIZE
> as array size, e.g.:
> 
> static void *virtqueue_split_pop(VirtQueue *vq, size_t sz)
> {
>     ...
>     hwaddr addr[VIRTQUEUE_MAX_SIZE];
>     struct iovec iov[VIRTQUEUE_MAX_SIZE];
>     ...
> }
> 
> > And as I am not too familiar with the virtio protocol, is that current limit
> > already visible to guest side? Because obviously it would make sense if I
> > change my kernel patches so that they automatically limit to whatever QEMU
> > supports instead of causing a hang.
> 
> Apparently the value of VIRTQUEUE_MAX_SIZE (the maximum amount of scatter
> gather lists or the maximum queue size ever possible) is not visible to guest.
> 
> I thought about making a hack to make the guest Linux kernel aware whether
> host side has the old limit of 1024 or rather the correct value 32768, but
> probably not worth it.
> 
> Best regards,
> Christian Schoenebeck
>