Re: [PATCH v2 0/3] virtio: increase VIRTQUEUE_MAX_SIZE to 32k

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Thu, Oct 07, 2021 at 02:51:55PM +0200, Christian Schoenebeck wrote:
> On Donnerstag, 7. Oktober 2021 07:23:59 CEST Stefan Hajnoczi wrote:
> > On Mon, Oct 04, 2021 at 09:38:00PM +0200, Christian Schoenebeck wrote:
> > > At the moment the maximum transfer size with virtio is limited to 4M
> > > (1024 * PAGE_SIZE). This series raises this limit to its maximum
> > > theoretical possible transfer size of 128M (32k pages) according to the
> > > virtio specs:
> > > 
> > > https://docs.oasis-open.org/virtio/virtio/v1.1/cs01/virtio-v1.1-cs01.html#
> > > x1-240006
> > Hi Christian,
> > I took a quick look at the code:
> > 
> > - The Linux 9p driver restricts descriptor chains to 128 elements
> >   (net/9p/trans_virtio.c:VIRTQUEUE_NUM)
> 
> Yes, that's the limitation that I am about to remove (WIP); current kernel 
> patches:
> https://lore.kernel.org/netdev/cover.1632327421.git.linux_oss@crudebyte.com/

I haven't read the patches yet but I'm concerned that today the driver
is pretty well-behaved and this new patch series introduces a spec
violation. Not fixing existing spec violations is okay, but adding new
ones is a red flag. I think we need to figure out a clean solution.

> > - The QEMU 9pfs code passes iovecs directly to preadv(2) and will fail
> >   with EINVAL when called with more than IOV_MAX iovecs
> >   (hw/9pfs/9p.c:v9fs_read())
> 
> Hmm, which makes me wonder why I never encountered this error during testing.
> 
> Most people will use the 9p qemu 'local' fs driver backend in practice, so 
> that v9fs_read() call would translate for most people to this implementation 
> on QEMU side (hw/9p/9p-local.c):
> 
> static ssize_t local_preadv(FsContext *ctx, V9fsFidOpenState *fs,
>                             const struct iovec *iov,
>                             int iovcnt, off_t offset)
> {
> #ifdef CONFIG_PREADV
>     return preadv(fs->fd, iov, iovcnt, offset);
> #else
>     int err = lseek(fs->fd, offset, SEEK_SET);
>     if (err == -1) {
>         return err;
>     } else {
>         return readv(fs->fd, iov, iovcnt);
>     }
> #endif
> }
> 
> > Unless I misunderstood the code, neither side can take advantage of the
> > new 32k descriptor chain limit?
> > 
> > Thanks,
> > Stefan
> 
> I need to check that when I have some more time. One possible explanation 
> might be that preadv() already has this wrapped into a loop in its 
> implementation to circumvent a limit like IOV_MAX. It might be another "it 
> works, but not portable" issue, but not sure.
>
> There are still a bunch of other issues I have to resolve. If you look at
> net/9p/client.c on kernel side, you'll notice that it basically does this ATM
> 
>     kmalloc(msize);
> 
> for every 9p request. So not only does it allocate much more memory for every 
> request than actually required (i.e. say 9pfs was mounted with msize=8M, then 
> a 9p request that actually would just need 1k would nevertheless allocate 8M), 
> but also it allocates > PAGE_SIZE, which obviously may fail at any time.

The PAGE_SIZE limitation sounds like a kmalloc() vs vmalloc() situation.

I saw zerocopy code in the 9p guest driver but didn't investigate when
it's used. Maybe that should be used for large requests (file
reads/writes)? virtio-blk/scsi don't memcpy data into a new buffer, they
directly access page cache or O_DIRECT pinned pages.

Stefan

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