From: Stefan Hajnoczi <stefanha@redhat.com>
To: Christian Schoenebeck <qemu_oss@crudebyte.com>
Cc: "Kevin Wolf" <kwolf@redhat.com>,
"Laurent Vivier" <lvivier@redhat.com>,
qemu-block@nongnu.org, "Michael S. Tsirkin" <mst@redhat.com>,
"Jason Wang" <jasowang@redhat.com>, "Amit Shah" <amit@kernel.org>,
"David Hildenbrand" <david@redhat.com>,
qemu-devel@nongnu.org, "Greg Kurz" <groug@kaod.org>,
virtio-fs@redhat.com, "Eric Auger" <eric.auger@redhat.com>,
"Hanna Reitz" <hreitz@redhat.com>,
"Gonglei (Arei)" <arei.gonglei@huawei.com>,
"Gerd Hoffmann" <kraxel@redhat.com>,
"Marc-André Lureau" <marcandre.lureau@redhat.com>,
"Paolo Bonzini" <pbonzini@redhat.com>,
"Fam Zheng" <fam@euphon.net>,
"Raphael Norwitz" <raphael.norwitz@nutanix.com>,
"Dr. David Alan Gilbert" <dgilbert@redhat.com>
Subject: Re: [PATCH v2 0/3] virtio: increase VIRTQUEUE_MAX_SIZE to 32k
Date: Thu, 7 Oct 2021 16:42:49 +0100 [thread overview]
Message-ID: <YV8VeaWiwD8DRFtz@stefanha-x1.localdomain> (raw)
In-Reply-To: <2233456.PtHKNz60go@silver>
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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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next prev parent reply other threads:[~2021-10-07 15:56 UTC|newest]
Thread overview: 48+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-10-04 19:38 [PATCH v2 0/3] virtio: increase VIRTQUEUE_MAX_SIZE to 32k Christian Schoenebeck
2021-10-04 19:38 ` [PATCH v2 1/3] virtio: turn VIRTQUEUE_MAX_SIZE into a variable Christian Schoenebeck
2021-10-05 7:36 ` Greg Kurz
2021-10-05 12:45 ` Stefan Hajnoczi
2021-10-05 13:15 ` Christian Schoenebeck
2021-10-05 15:10 ` Stefan Hajnoczi
2021-10-05 16:32 ` Christian Schoenebeck
2021-10-06 11:06 ` Stefan Hajnoczi
2021-10-06 12:50 ` Christian Schoenebeck
2021-10-06 14:42 ` Stefan Hajnoczi
2021-10-07 13:09 ` Christian Schoenebeck
2021-10-07 15:18 ` Stefan Hajnoczi
2021-10-08 14:48 ` Christian Schoenebeck
2021-10-04 19:38 ` [PATCH v2 2/3] virtio: increase VIRTQUEUE_MAX_SIZE to 32k Christian Schoenebeck
2021-10-05 7:16 ` Michael S. Tsirkin
2021-10-05 7:35 ` Greg Kurz
2021-10-05 11:17 ` Christian Schoenebeck
2021-10-05 11:24 ` Michael S. Tsirkin
2021-10-05 12:01 ` Christian Schoenebeck
2021-10-04 19:38 ` [PATCH v2 3/3] virtio-9p-device: switch to 32k max. transfer size Christian Schoenebeck
2021-10-05 7:38 ` [PATCH v2 0/3] virtio: increase VIRTQUEUE_MAX_SIZE to 32k David Hildenbrand
2021-10-05 11:10 ` Christian Schoenebeck
2021-10-05 11:19 ` Michael S. Tsirkin
2021-10-05 11:43 ` Christian Schoenebeck
2021-10-07 5:23 ` Stefan Hajnoczi
2021-10-07 12:51 ` Christian Schoenebeck
2021-10-07 15:42 ` Stefan Hajnoczi [this message]
2021-10-08 7:25 ` Greg Kurz
2021-10-08 14:24 ` Christian Schoenebeck
2021-10-08 16:08 ` Christian Schoenebeck
2021-10-21 15:39 ` Christian Schoenebeck
2021-10-25 10:30 ` Stefan Hajnoczi
2021-10-25 15:03 ` Christian Schoenebeck
2021-10-28 9:00 ` Stefan Hajnoczi
2021-11-01 20:29 ` Christian Schoenebeck
2021-11-03 11:33 ` Stefan Hajnoczi
2021-11-04 14:41 ` Christian Schoenebeck
2021-11-09 10:56 ` Stefan Hajnoczi
2021-11-09 13:09 ` Christian Schoenebeck
2021-11-10 10:05 ` Stefan Hajnoczi
2021-11-10 13:14 ` Christian Schoenebeck
2021-11-10 15:14 ` Stefan Hajnoczi
2021-11-10 15:53 ` Christian Schoenebeck
2021-11-11 16:31 ` Stefan Hajnoczi
2021-11-11 17:54 ` Christian Schoenebeck
2021-11-15 11:54 ` Stefan Hajnoczi
2021-11-15 14:32 ` Christian Schoenebeck
2021-11-16 11:13 ` Stefan Hajnoczi
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