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

[Christian Schoenebeck <qemu_oss@crudebyte.com>]

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/

> - 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.

With those kernel patches above and QEMU being patched with these series as 
well, I can go above 4M msize now, and the test system runs stable if 9pfs was 
mounted with an msize not being "too high". If I try to mount 9pfs with msize 
being very high, the upper described kmalloc() issue would kick in and cause 
an immediate kernel oops when mounting. So that's a high priority issue that I 
still need to resolve.

Best regards,
Christian Schoenebeck