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_...@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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