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Tsirkin" , Jason Wang , Amit Shah , David Hildenbrand , Greg Kurz , virtio-fs@redhat.com, Eric Auger , Hanna Reitz , "Gonglei (Arei)" , Gerd Hoffmann , Paolo Bonzini , =?ISO-8859-1?Q?Marc=2DAndr=E9?= Lureau , Fam Zheng , Raphael Norwitz , "Dr. David Alan Gilbert" Subject: Re: [PATCH v2 0/3] virtio: increase VIRTQUEUE_MAX_SIZE to 32k Date: Tue, 09 Nov 2021 14:09:59 +0100 Message-ID: <25571471.tMsSMU6axZ@silver> In-Reply-To: References: <10421947.PG2v2feK4y@silver> MIME-Version: 1.0 Content-Transfer-Encoding: 7Bit Content-Type: text/plain; charset="us-ascii" Received-SPF: pass client-ip=5.189.157.229; envelope-from=qemu_oss@crudebyte.com; helo=kylie.crudebyte.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Dienstag, 9. November 2021 11:56:35 CET Stefan Hajnoczi wrote: > On Thu, Nov 04, 2021 at 03:41:23PM +0100, Christian Schoenebeck wrote: > > On Mittwoch, 3. November 2021 12:33:33 CET Stefan Hajnoczi wrote: > > > On Mon, Nov 01, 2021 at 09:29:26PM +0100, Christian Schoenebeck wrote: > > > > On Donnerstag, 28. Oktober 2021 11:00:48 CET Stefan Hajnoczi wrote: > > > > > On Mon, Oct 25, 2021 at 05:03:25PM +0200, Christian Schoenebeck wrote: > > > > > > On Montag, 25. Oktober 2021 12:30:41 CEST Stefan Hajnoczi wrote: > > > > > > > On Thu, Oct 21, 2021 at 05:39:28PM +0200, Christian Schoenebeck wrote: > > > > > > > > On Freitag, 8. Oktober 2021 18:08:48 CEST Christian Schoenebeck wrote: > > > > > > > > > On Freitag, 8. Oktober 2021 16:24:42 CEST Christian Schoenebeck wrote: > > > > > > > > > > On Freitag, 8. Oktober 2021 09:25:33 CEST Greg Kurz wrote: > > > > > > > > > > > On Thu, 7 Oct 2021 16:42:49 +0100 > > > > > > > > > > > > > > > > > > > > > > Stefan Hajnoczi wrote: > > > > > > > > > > > > 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/c > > > > > > > > > > > > > > > s01/ > > > > > > > > > > > > > > > virt > > > > > > > > > > > > > > > io-v > > > > > > > > > > > > > > > 1.1- > > > > > > > > > > > > > > > cs > > > > > > > > > > > > > > > 01 > > > > > > > > > > > > > > > .html# > > > > > > > > > > > > > > > x1-240006 > > > > > > > > > > > > > > > > > > > > > > > > > > > > Hi Christian, > > > > > > > > > > > > > > > > > > > > > > > > > I took a quick look at the code: > > > > > > > > > > > Hi, > > > > > > > > > > > > > > > > > > > > > > Thanks Stefan for sharing virtio expertise and helping > > > > > > > > > > > Christian > > > > > > > > > > > ! > > > > > > > > > > > > > > > > > > > > > > > > > - 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. > > > > > > > > > > > > > linu > > > > > > > > > > > > > x_os > > > > > > > > > > > > > s@cr > > > > > > > > > > > > > udeb > > > > > > > > > > > > > yt > > > > > > > > > > > > > e. > > > > > > > > > > > > > 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. > > > > > > > > > > > > > > > > > > > > Nobody has reviewed the kernel patches yet. My main > > > > > > > > > > concern > > > > > > > > > > therefore > > > > > > > > > > actually is that the kernel patches are already too > > > > > > > > > > complex, > > > > > > > > > > because > > > > > > > > > > the > > > > > > > > > > current situation is that only Dominique is handling 9p > > > > > > > > > > patches on > > > > > > > > > > kernel > > > > > > > > > > side, and he barely has time for 9p anymore. > > > > > > > > > > > > > > > > > > > > Another reason for me to catch up on reading current > > > > > > > > > > kernel > > > > > > > > > > code > > > > > > > > > > and > > > > > > > > > > stepping in as reviewer of 9p on kernel side ASAP, > > > > > > > > > > independent > > > > > > > > > > of > > > > > > > > > > this > > > > > > > > > > issue. > > > > > > > > > > > > > > > > > > > > As for current kernel patches' complexity: I can certainly > > > > > > > > > > drop > > > > > > > > > > patch > > > > > > > > > > 7 > > > > > > > > > > entirely as it is probably just overkill. Patch 4 is then > > > > > > > > > > the > > > > > > > > > > biggest > > > > > > > > > > chunk, I have to see if I can simplify it, and whether it > > > > > > > > > > would > > > > > > > > > > make > > > > > > > > > > sense to squash with patch 3. > > > > > > > > > > > > > > > > > > > > > > > > - 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); > > > > > > > > > > > > > > > > > > > > > > Note that this is done twice : once for the T message > > > > > > > > > > > (client > > > > > > > > > > > request) > > > > > > > > > > > and > > > > > > > > > > > once for the R message (server answer). The 9p driver > > > > > > > > > > > could > > > > > > > > > > > adjust > > > > > > > > > > > the > > > > > > > > > > > size > > > > > > > > > > > of the T message to what's really needed instead of > > > > > > > > > > > allocating > > > > > > > > > > > the > > > > > > > > > > > full > > > > > > > > > > > msize. R message size is not known though. > > > > > > > > > > > > > > > > > > > > Would it make sense adding a second virtio ring, dedicated > > > > > > > > > > to > > > > > > > > > > server > > > > > > > > > > responses to solve this? IIRC 9p server already calculates > > > > > > > > > > appropriate > > > > > > > > > > exact sizes for each response type. So server could just > > > > > > > > > > push > > > > > > > > > > space > > > > > > > > > > that's > > > > > > > > > > really needed for its responses. > > > > > > > > > > > > > > > > > > > > > > > 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. > > > > > > > > > > > > > > > > > > > > Hu, I didn't even consider vmalloc(). I just tried the > > > > > > > > > > kvmalloc() > > > > > > > > > > wrapper > > > > > > > > > > as a quick & dirty test, but it crashed in the same way as > > > > > > > > > > kmalloc() > > > > > > > > > > with > > > > > > > > > > large msize values immediately on mounting: > > > > > > > > > > > > > > > > > > > > diff --git a/net/9p/client.c b/net/9p/client.c > > > > > > > > > > index a75034fa249b..cfe300a4b6ca 100644 > > > > > > > > > > --- a/net/9p/client.c > > > > > > > > > > +++ b/net/9p/client.c > > > > > > > > > > @@ -227,15 +227,18 @@ static int parse_opts(char *opts, > > > > > > > > > > struct > > > > > > > > > > p9_client > > > > > > > > > > *clnt) > > > > > > > > > > > > > > > > > > > > static int p9_fcall_init(struct p9_client *c, struct > > > > > > > > > > p9_fcall > > > > > > > > > > *fc, > > > > > > > > > > > > > > > > > > > > int alloc_msize) > > > > > > > > > > > > > > > > > > > > { > > > > > > > > > > > > > > > > > > > > - if (likely(c->fcall_cache) && alloc_msize == > > > > > > > > > > c->msize) > > > > > > > > > > { > > > > > > > > > > + //if (likely(c->fcall_cache) && alloc_msize == > > > > > > > > > > c->msize) { > > > > > > > > > > + if (false) { > > > > > > > > > > > > > > > > > > > > fc->sdata = > > > > > > > > > > kmem_cache_alloc(c->fcall_cache, > > > > > > > > > > GFP_NOFS); > > > > > > > > > > fc->cache = c->fcall_cache; > > > > > > > > > > > > > > > > > > > > } else { > > > > > > > > > > > > > > > > > > > > - fc->sdata = kmalloc(alloc_msize, > > > > > > > > > > GFP_NOFS); > > > > > > > > > > + fc->sdata = kvmalloc(alloc_msize, > > > > > > > > > > GFP_NOFS); > > > > > > > > > > > > > > > > > > Ok, GFP_NOFS -> GFP_KERNEL did the trick. > > > > > > > > > > > > > > > > > > Now I get: > > > > > > > > > virtio: bogus descriptor or out of resources > > > > > > > > > > > > > > > > > > So, still some work ahead on both ends. > > > > > > > > > > > > > > > > Few hacks later (only changes on 9p client side) I got this > > > > > > > > running > > > > > > > > stable > > > > > > > > now. The reason for the virtio error above was that kvmalloc() > > > > > > > > returns > > > > > > > > a > > > > > > > > non-logical kernel address for any kvmalloc(>4M), i.e. an > > > > > > > > address > > > > > > > > that > > > > > > > > is > > > > > > > > inaccessible from host side, hence that "bogus descriptor" > > > > > > > > message > > > > > > > > by > > > > > > > > QEMU. > > > > > > > > So I had to split those linear 9p client buffers into sparse > > > > > > > > ones > > > > > > > > (set > > > > > > > > of > > > > > > > > individual pages). > > > > > > > > > > > > > > > > I tested this for some days with various virtio transmission > > > > > > > > sizes > > > > > > > > and > > > > > > > > it > > > > > > > > works as expected up to 128 MB (more precisely: 128 MB read > > > > > > > > space > > > > > > > > + > > > > > > > > 128 MB > > > > > > > > write space per virtio round trip message). > > > > > > > > > > > > > > > > I did not encounter a show stopper for large virtio > > > > > > > > transmission > > > > > > > > sizes > > > > > > > > (4 MB ... 128 MB) on virtio level, neither as a result of > > > > > > > > testing, > > > > > > > > nor > > > > > > > > after reviewing the existing code. > > > > > > > > > > > > > > > > About IOV_MAX: that's apparently not an issue on virtio level. > > > > > > > > Most of > > > > > > > > the > > > > > > > > iovec code, both on Linux kernel side and on QEMU side do not > > > > > > > > have > > > > > > > > this > > > > > > > > limitation. It is apparently however indeed a limitation for > > > > > > > > userland > > > > > > > > apps > > > > > > > > calling the Linux kernel's syscalls yet. > > > > > > > > > > > > > > > > Stefan, as it stands now, I am even more convinced that the > > > > > > > > upper > > > > > > > > virtio > > > > > > > > transmission size limit should not be squeezed into the queue > > > > > > > > size > > > > > > > > argument of virtio_add_queue(). Not because of the previous > > > > > > > > argument > > > > > > > > that > > > > > > > > it would waste space (~1MB), but rather because they are two > > > > > > > > different > > > > > > > > things. To outline this, just a quick recap of what happens > > > > > > > > exactly > > > > > > > > when > > > > > > > > a bulk message is pushed over the virtio wire (assuming virtio > > > > > > > > "split" > > > > > > > > layout here): > > > > > > > > > > > > > > > > ---------- [recap-start] ---------- > > > > > > > > > > > > > > > > For each bulk message sent guest <-> host, exactly *one* of > > > > > > > > the > > > > > > > > pre-allocated descriptors is taken and placed (subsequently) > > > > > > > > into > > > > > > > > exactly > > > > > > > > *one* position of the two available/used ring buffers. The > > > > > > > > actual > > > > > > > > descriptor table though, containing all the DMA addresses of > > > > > > > > the > > > > > > > > message > > > > > > > > bulk data, is allocated just in time for each round trip > > > > > > > > message. > > > > > > > > Say, > > > > > > > > it > > > > > > > > is the first message sent, it yields in the following > > > > > > > > structure: > > > > > > > > > > > > > > > > Ring Buffer Descriptor Table Bulk Data Pages > > > > > > > > > > > > > > > > +-+ +-+ +-----------------+ > > > > > > > > > > > > > > > > |D|------------->|d|---------->| Bulk data block | > > > > > > > > > > > > > > > > +-+ |d|--------+ +-----------------+ > > > > > > > > > > > > > > > > | | |d|------+ | > > > > > > > > > > > > > > > > +-+ . | | +-----------------+ > > > > > > > > > > > > > > > > | | . | +->| Bulk data block | > > > > > > > > > > > > > > > > . . | +-----------------+ > > > > > > > > . |d|-+ | > > > > > > > > . +-+ | | +-----------------+ > > > > > > > > > > > > > > > > | | | +--->| Bulk data block | > > > > > > > > > > > > > > > > +-+ | +-----------------+ > > > > > > > > > > > > > > > > | | | . > > > > > > > > > > > > > > > > +-+ | . > > > > > > > > > > > > > > > > | . > > > > > > > > | > > > > > > > > | +-----------------+ > > > > > > > > > > > > > > > > +-------->| Bulk data block | > > > > > > > > > > > > > > > > +-----------------+ > > > > > > > > > > > > > > > > Legend: > > > > > > > > D: pre-allocated descriptor > > > > > > > > d: just in time allocated descriptor > > > > > > > > -->: memory pointer (DMA) > > > > > > > > > > > > > > > > The bulk data blocks are allocated by the respective device > > > > > > > > driver > > > > > > > > above > > > > > > > > virtio subsystem level (guest side). > > > > > > > > > > > > > > > > There are exactly as many descriptors pre-allocated (D) as the > > > > > > > > size of > > > > > > > > a > > > > > > > > ring buffer. > > > > > > > > > > > > > > > > A "descriptor" is more or less just a chainable DMA memory > > > > > > > > pointer; > > > > > > > > defined > > > > > > > > as: > > > > > > > > > > > > > > > > /* Virtio ring descriptors: 16 bytes. These can chain > > > > > > > > together > > > > > > > > via > > > > > > > > "next". */ struct vring_desc { > > > > > > > > > > > > > > > > /* Address (guest-physical). */ > > > > > > > > __virtio64 addr; > > > > > > > > /* Length. */ > > > > > > > > __virtio32 len; > > > > > > > > /* The flags as indicated above. */ > > > > > > > > __virtio16 flags; > > > > > > > > /* We chain unused descriptors via this, too */ > > > > > > > > __virtio16 next; > > > > > > > > > > > > > > > > }; > > > > > > > > > > > > > > > > There are 2 ring buffers; the "available" ring buffer is for > > > > > > > > sending a > > > > > > > > message guest->host (which will transmit DMA addresses of > > > > > > > > guest > > > > > > > > allocated > > > > > > > > bulk data blocks that are used for data sent to device, and > > > > > > > > separate > > > > > > > > guest allocated bulk data blocks that will be used by host > > > > > > > > side to > > > > > > > > place > > > > > > > > its response bulk data), and the "used" ring buffer is for > > > > > > > > sending > > > > > > > > host->guest to let guest know about host's response and that > > > > > > > > it > > > > > > > > could > > > > > > > > now > > > > > > > > safely consume and then deallocate the bulk data blocks > > > > > > > > subsequently. > > > > > > > > > > > > > > > > ---------- [recap-end] ---------- > > > > > > > > > > > > > > > > So the "queue size" actually defines the ringbuffer size. It > > > > > > > > does > > > > > > > > not > > > > > > > > define the maximum amount of descriptors. The "queue size" > > > > > > > > rather > > > > > > > > defines > > > > > > > > how many pending messages can be pushed into either one > > > > > > > > ringbuffer > > > > > > > > before > > > > > > > > the other side would need to wait until the counter side would > > > > > > > > step up > > > > > > > > (i.e. ring buffer full). > > > > > > > > > > > > > > > > The maximum amount of descriptors (what VIRTQUEUE_MAX_SIZE > > > > > > > > actually > > > > > > > > is) > > > > > > > > OTOH defines the max. bulk data size that could be transmitted > > > > > > > > with > > > > > > > > each > > > > > > > > virtio round trip message. > > > > > > > > > > > > > > > > And in fact, 9p currently handles the virtio "queue size" as > > > > > > > > directly > > > > > > > > associative with its maximum amount of active 9p requests the > > > > > > > > server > > > > > > > > could > > > > > > > > > > > > > > > > handle simultaniously: > > > > > > > > hw/9pfs/9p.h:#define MAX_REQ 128 > > > > > > > > hw/9pfs/9p.h: V9fsPDU pdus[MAX_REQ]; > > > > > > > > hw/9pfs/virtio-9p-device.c: v->vq = > > > > > > > > virtio_add_queue(vdev, > > > > > > > > MAX_REQ, > > > > > > > > > > > > > > > > handle_9p_output); > > > > > > > > > > > > > > > > So if I would change it like this, just for the purpose to > > > > > > > > increase > > > > > > > > the > > > > > > > > max. virtio transmission size: > > > > > > > > > > > > > > > > --- a/hw/9pfs/virtio-9p-device.c > > > > > > > > +++ b/hw/9pfs/virtio-9p-device.c > > > > > > > > @@ -218,7 +218,7 @@ static void > > > > > > > > virtio_9p_device_realize(DeviceState > > > > > > > > *dev, > > > > > > > > Error **errp)> > > > > > > > > > > > > > > > > v->config_size = sizeof(struct virtio_9p_config) + > > > > > > > > strlen(s->fsconf.tag); > > > > > > > > virtio_init(vdev, "virtio-9p", VIRTIO_ID_9P, > > > > > > > > v->config_size, > > > > > > > > > > > > > > > > VIRTQUEUE_MAX_SIZE); > > > > > > > > > > > > > > > > - v->vq = virtio_add_queue(vdev, MAX_REQ, > > > > > > > > handle_9p_output); > > > > > > > > + v->vq = virtio_add_queue(vdev, 32*1024, > > > > > > > > handle_9p_output); > > > > > > > > > > > > > > > > } > > > > > > > > > > > > > > > > Then it would require additional synchronization code on both > > > > > > > > ends > > > > > > > > and > > > > > > > > therefore unnecessary complexity, because it would now be > > > > > > > > possible > > > > > > > > that > > > > > > > > more requests are pushed into the ringbuffer than server could > > > > > > > > handle. > > > > > > > > > > > > > > > > There is one potential issue though that probably did justify > > > > > > > > the > > > > > > > > "don't > > > > > > > > exceed the queue size" rule: > > > > > > > > > > > > > > > > ATM the descriptor table is allocated (just in time) as *one* > > > > > > > > continuous > > > > > > > > buffer via kmalloc_array(): > > > > > > > > https://github.com/torvalds/linux/blob/2f111a6fd5b5297b4e92f53 > > > > > > > > 798c > > > > > > > > a086 > > > > > > > > f7c7 > > > > > > > > d33a4/drivers/virtio/virtio_ring.c#L440 > > > > > > > > > > > > > > > > So assuming transmission size of 2 * 128 MB that > > > > > > > > kmalloc_array() > > > > > > > > call > > > > > > > > would > > > > > > > > yield in kmalloc(1M) and the latter might fail if guest had > > > > > > > > highly > > > > > > > > fragmented physical memory. For such kind of error case there > > > > > > > > is > > > > > > > > currently a fallback path in virtqueue_add_split() that would > > > > > > > > then > > > > > > > > use > > > > > > > > the required amount of pre-allocated descriptors instead: > > > > > > > > https://github.com/torvalds/linux/blob/2f111a6fd5b5297b4e92f53 > > > > > > > > 798c > > > > > > > > a086 > > > > > > > > f7c7 > > > > > > > > d33a4/drivers/virtio/virtio_ring.c#L525 > > > > > > > > > > > > > > > > That fallback recovery path would no longer be viable if the > > > > > > > > queue > > > > > > > > size > > > > > > > > was > > > > > > > > exceeded. There would be alternatives though, e.g. by allowing > > > > > > > > to > > > > > > > > chain > > > > > > > > indirect descriptor tables (currently prohibited by the virtio > > > > > > > > specs). > > > > > > > > > > > > > > Making the maximum number of descriptors independent of the > > > > > > > queue > > > > > > > size > > > > > > > requires a change to the VIRTIO spec since the two values are > > > > > > > currently > > > > > > > explicitly tied together by the spec. > > > > > > > > > > > > Yes, that's what the virtio specs say. But they don't say why, nor > > > > > > did > > > > > > I > > > > > > hear a reason in this dicussion. > > > > > > > > > > > > That's why I invested time reviewing current virtio implementation > > > > > > and > > > > > > specs, as well as actually testing exceeding that limit. And as I > > > > > > outlined in detail in my previous email, I only found one > > > > > > theoretical > > > > > > issue that could be addressed though. > > > > > > > > > > I agree that there is a limitation in the VIRTIO spec, but violating > > > > > the > > > > > spec isn't an acceptable solution: > > > > > > > > > > 1. QEMU and Linux aren't the only components that implement VIRTIO. > > > > > You > > > > > > > > > > cannot make assumptions about their implementations because it > > > > > may > > > > > break spec-compliant implementations that you haven't looked at. > > > > > > > > > > Your patches weren't able to increase Queue Size because some > > > > > device > > > > > implementations break when descriptor chains are too long. This > > > > > shows > > > > > there is a practical issue even in QEMU. > > > > > > > > > > 2. The specific spec violation that we discussed creates the problem > > > > > > > > > > that drivers can no longer determine the maximum description > > > > > chain > > > > > length. This in turn will lead to more implementation-specific > > > > > assumptions being baked into drivers and cause problems with > > > > > interoperability and future changes. > > > > > > > > > > The spec needs to be extended instead. I included an idea for how to > > > > > do > > > > > that below. > > > > > > > > Sure, I just wanted to see if there was a non-neglectable "hard" show > > > > stopper per se that I probably haven't seen yet. I have not questioned > > > > aiming a clean solution. > > > > > > > > Thanks for the clarification! > > > > > > > > > > > Before doing that, are there benchmark results showing that 1 MB > > > > > > > vs > > > > > > > 128 > > > > > > > MB produces a performance improvement? I'm asking because if > > > > > > > performance > > > > > > > with 1 MB is good then you can probably do that without having > > > > > > > to > > > > > > > change > > > > > > > VIRTIO and also because it's counter-intuitive that 9p needs 128 > > > > > > > MB > > > > > > > for > > > > > > > good performance when it's ultimately implemented on top of disk > > > > > > > and > > > > > > > network I/O that have lower size limits. > > > > > > > > > > > > First some numbers, linear reading a 12 GB file: > > > > > > > > > > > > msize average notes > > > > > > > > > > > > 8 kB 52.0 MB/s default msize of Linux kernel > > > > > 128 kB 624.8 MB/s default msize of Linux kernel >=v5.15 > > > > > > 512 kB 1961 MB/s current max. msize with any Linux kernel > > > > > > <=v5.15 > > > > > > 1 MB 2551 MB/s this msize would already violate virtio > > > > > > specs > > > > > > 2 MB 2521 MB/s this msize would already violate virtio > > > > > > specs > > > > > > 4 MB 2628 MB/s planned max. msize of my current kernel > > > > > > patches > > > > > > [1] > > > > > > > > > > How many descriptors are used? 4 MB can be covered by a single > > > > > descriptor if the data is physically contiguous in memory, so this > > > > > data > > > > > doesn't demonstrate a need for more descriptors. > > > > > > > > No, in the last couple years there was apparently no kernel version > > > > that > > > > used just one descriptor, nor did my benchmarked version. Even though > > > > the > > > > Linux 9p client uses (yet) simple linear buffers (contiguous physical > > > > memory) on 9p client level, these are however split into PAGE_SIZE > > > > chunks > > > > by function pack_sg_list() [1] before being fed to virtio level: > > > > > > > > static unsigned int rest_of_page(void *data) > > > > { > > > > > > > > return PAGE_SIZE - offset_in_page(data); > > > > > > > > } > > > > ... > > > > static int pack_sg_list(struct scatterlist *sg, int start, > > > > > > > > int limit, char *data, int count) > > > > > > > > { > > > > > > > > int s; > > > > int index = start; > > > > > > > > while (count) { > > > > > > > > s = rest_of_page(data); > > > > ... > > > > sg_set_buf(&sg[index++], data, s); > > > > count -= s; > > > > data += s; > > > > > > > > } > > > > ... > > > > > > > > } > > > > > > > > [1] > > > > https://github.com/torvalds/linux/blob/19901165d90fdca1e57c9baa0d5b4c6 > > > > 3d1 > > > > 5c476a/net/9p/trans_virtio.c#L171 > > > > > > > > So when sending 4MB over virtio wire, it would yield in 1k descriptors > > > > ATM. > > > > > > > > I have wondered about this before, but did not question it, because > > > > due to > > > > the cross-platform nature I couldn't say for certain whether that's > > > > probably needed somewhere. I mean for the case virtio-PCI I know for > > > > sure > > > > that one descriptor (i.e. >PAGE_SIZE) would be fine, but I don't know > > > > if > > > > that applies to all buses and architectures. > > > > > > VIRTIO does not limit descriptor the descriptor len field to PAGE_SIZE, > > > so I don't think there is a limit at the VIRTIO level. > > > > So you are viewing this purely from virtio specs PoV: in the sense, if it > > is not prohibited by the virtio specs, then it should work. Maybe. > > Limitations must be specified either in the 9P protocol or the VIRTIO > specification. Drivers and devices will not be able to operate correctly > if there are limitations that aren't covered by the specs. > > Do you have something in mind that isn't covered by the specs? Not sure whether that's something that should be specified by the virtio specs, probably not. I simply do not know if there was any bus or architecture that would have a limitation for max. size for a memory block passed per one DMA address. > > > If this function coalesces adjacent pages then the descriptor chain > > > length issues could be reduced. > > > > > > > > > But again, this is not just about performance. My conclusion as > > > > > > described > > > > > > in my previous email is that virtio currently squeezes > > > > > > > > > > > > "max. simultanious amount of bulk messages" > > > > > > > > > > > > vs. > > > > > > > > > > > > "max. bulk data transmission size per bulk messaage" > > > > > > > > > > > > into the same configuration parameter, which is IMO inappropriate > > > > > > and > > > > > > hence > > > > > > splitting them into 2 separate parameters when creating a queue > > > > > > makes > > > > > > sense, independent of the performance benchmarks. > > > > > > > > > > > > [1] > > > > > > https://lore.kernel.org/netdev/cover.1632327421.git.linux_oss@crud > > > > > > ebyt > > > > > > e.c > > > > > > om/ > > > > > > > > > > Some devices effectively already have this because the device > > > > > advertises > > > > > a maximum number of descriptors via device-specific mechanisms like > > > > > the > > > > > struct virtio_blk_config seg_max field. But today these fields can > > > > > only > > > > > reduce the maximum descriptor chain length because the spec still > > > > > limits > > > > > the length to Queue Size. > > > > > > > > > > We can build on this approach to raise the length above Queue Size. > > > > > This > > > > > approach has the advantage that the maximum number of segments isn't > > > > > per > > > > > device or per virtqueue, it's fine-grained. If the device supports > > > > > two > > > > > requests types then different max descriptor chain limits could be > > > > > given > > > > > for them by introducing two separate configuration space fields. > > > > > > > > > > Here are the corresponding spec changes: > > > > > > > > > > 1. A new feature bit called VIRTIO_RING_F_LARGE_INDIRECT_DESC is > > > > > added > > > > > > > > > > to indicate that indirect descriptor table size and maximum > > > > > descriptor chain length are not limited by Queue Size value. > > > > > (Maybe > > > > > there still needs to be a limit like 2^15?) > > > > > > > > Sounds good to me! > > > > > > > > AFAIK it is effectively limited to 2^16 because of vring_desc->next: > > > > > > > > /* Virtio ring descriptors: 16 bytes. These can chain together via > > > > "next". */ struct vring_desc { > > > > > > > > /* Address (guest-physical). */ > > > > __virtio64 addr; > > > > /* Length. */ > > > > __virtio32 len; > > > > /* The flags as indicated above. */ > > > > __virtio16 flags; > > > > /* We chain unused descriptors via this, too */ > > > > __virtio16 next; > > > > > > > > }; > > > > > > Yes, Split Virtqueues have a fundamental limit on indirect table size > > > due to the "next" field. Packed Virtqueue descriptors don't have a > > > "next" field so descriptor chains could be longer in theory (currently > > > forbidden by the spec). > > > > > > > > One thing that's messy is that we've been discussing the maximum > > > > > descriptor chain length but 9p has the "msize" concept, which isn't > > > > > aware of contiguous memory. It may be necessary to extend the 9p > > > > > driver > > > > > code to size requests not just according to their length in bytes > > > > > but > > > > > also according to the descriptor chain length. That's how the Linux > > > > > block layer deals with queue limits (struct queue_limits > > > > > max_segments vs > > > > > max_hw_sectors). > > > > > > > > Hmm, can't follow on that one. For what should that be needed in case > > > > of > > > > 9p? My plan was to limit msize by 9p client simply at session start to > > > > whatever is the max. amount virtio descriptors supported by host and > > > > using PAGE_SIZE as size per descriptor, because that's what 9p client > > > > actually does ATM (see above). So you think that should be changed to > > > > e.g. just one descriptor for 4MB, right? > > > > > > Limiting msize to the 9p transport device's maximum number of > > > descriptors is conservative (i.e. 128 descriptors = 512 KB msize) > > > because it doesn't take advantage of contiguous memory. I suggest > > > leaving msize alone, adding a separate limit at which requests are split > > > according to the maximum descriptor chain length, and tweaking > > > pack_sg_list() to coalesce adjacent pages. > > > > > > That way msize can be large without necessarily using lots of > > > descriptors (depending on the memory layout). > > > > That was actually a tempting solution. Because it would neither require > > changes to the virtio specs (at least for a while) and it would also work > > with older QEMU versions. And for that pack_sg_list() portion of the code > > it would work well and easy as the buffer passed to pack_sg_list() is > > contiguous already. > > > > However I just realized for the zero-copy version of the code that would > > be > > more tricky. The ZC version already uses individual pages (struct page, > > hence PAGE_SIZE each) which are pinned, i.e. it uses pack_sg_list_p() [1] > > in combination with p9_get_mapped_pages() [2] > > > > [1] > > https://github.com/torvalds/linux/blob/7ddb58cb0ecae8e8b6181d736a87667cc9 > > ab8389/net/9p/trans_virtio.c#L218 [2] > > https://github.com/torvalds/linux/blob/7ddb58cb0ecae8e8b6181d736a87667cc9 > > ab8389/net/9p/trans_virtio.c#L309 > > > > So that would require much more work and code trying to sort and coalesce > > individual pages to contiguous physical memory for the sake of reducing > > virtio descriptors. And there is no guarantee that this is even possible. > > The kernel may simply return a non-contiguous set of pages which would > > eventually end up exceeding the virtio descriptor limit again. > > Order must be preserved so pages cannot be sorted by physical address. > How about simply coalescing when pages are adjacent? It would help, but not solve the issue we are talking about here: if 99% of the cases could successfully merge descriptors to stay below the descriptor count limit, but in 1% of the cases it could not, then this still construes a severe runtime issue that could trigger at any time. > > So looks like it was probably still easier and realistic to just add > > virtio > > capabilities for now for allowing to exceed current descriptor limit. > > I'm still not sure why virtio-net, virtio-blk, virtio-fs, etc perform > fine under today's limits while virtio-9p needs a much higher limit to > achieve good performance. Maybe there is an issue in a layer above the > vring that's causing the virtio-9p performance you've observed? Are you referring to (somewhat) recent benchmarks when saying those would all still perform fine today? Vivek was running detailed benchmarks for virtiofs vs. 9p: https://lists.gnu.org/archive/html/qemu-devel/2020-12/msg02704.html For the virtio aspect discussed here, only the benchmark configurations without cache are relevant (9p-none, vtfs-none) and under this aspect the situation seems to be quite similar between 9p and virtio-fs. You'll also note once DAX is enabled (vtfs-none-dax) that apparently boosts virtio-fs performance significantly, which however seems to corelate to numbers when I am running 9p with msize > 300k. Note: Vivek was presumably running 9p effecively with msize=300k, as this was the kernel limitation at that time. To bring things into relation: there are known performance aspects in 9p that can be improved, yes, both on Linux kernel side and on 9p server side in QEMU. For instance 9p server uses coroutines [1] and currently dispatches between worker thread(s) and main thread too often per request (partly addressed already [2], but still WIP), which accumulates to overall latency. But Vivek was actually using a 9p patch here which disabled coroutines entirely, which suggests that the virtio transmission size limit still represents a bottleneck. [1] https://wiki.qemu.org/Documentation/9p#Coroutines [2] https://wiki.qemu.org/Documentation/9p#Implementation_Plans Best regards, Christian Schoenebeck From mboxrd@z Thu Jan 1 00:00:00 1970 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=crudebyte.com; s=kylie; h=Content-Type:Content-Transfer-Encoding: MIME-Version:References:In-Reply-To:Message-ID:Date:Subject:Cc:To:From: Content-ID:Content-Description; bh=xz2IP+KOjlAlxq5Su+A16FbxMegIAN2cNVhVYSR1wjY=; b=L1Hh7/Gvmps7+StxZb8mRZI9sj SOLkj0sQj833n93Z1bUGzd1F6r6CUuLJ4GPsPDm5AEZQEMoo6okZsx1RymN9VuH2TNsvkT7cIJ9BM dgD5iQj7NihrcBDmkD0US1MEdiWLofAsd4rfbjfGYoOW2LN1usCiabN4dyhjuat5El+zwbGGcu/EJ tCnVntEs1MNerdNxvbAVRuzZVsvfr++meUkbPGuw3f/ljT6ryVrrWd04get0f7GWZvcbwM7L6T6jI JdrjXJc5XAKVpjjjkZQptRCLZBpxTsOjrS9yJ+9r3Fro3ZriRW9GScNQn7TGjJvLJKedH4ZdUzt70 ogZkrtfEf5JXDJNlRRvR6Qz3DRzrY41yUB0lc3jwwJCX9vLBXPmLnx5wJAwaeu/Q+4H7Ea8H9ENdx gF8qMwRhpzbR4G51zxoRtfmG1HprF+qULszC7at5fh+yjg+qxRQO7pgusaDHkuwOEnIcJ1sQknDnD kUYyMzTTM0ou4mk0Cc1woBLeI7A6jxX4VEIEdFbgkE3A3BYwkw5cLJFW5ESTsaoJCq7M3gp7LacXh o810r+vZgl004Yoa7Xb2FQHdurNJ4Y1xB0si9WIxl+RfRAtkHhbK+SKCsNRp6H+UlNoWVP/MZ4rOj soKueI70BlQtVteYrnJQCSz4U+dNXHi177VxfFNnw= From: Christian Schoenebeck Date: Tue, 09 Nov 2021 14:09:59 +0100 Message-ID: <25571471.tMsSMU6axZ@silver> In-Reply-To: References: <10421947.PG2v2feK4y@silver> MIME-Version: 1.0 Content-Transfer-Encoding: 7Bit Content-Type: text/plain; charset="us-ascii" Subject: Re: [Virtio-fs] [PATCH v2 0/3] virtio: increase VIRTQUEUE_MAX_SIZE to 32k List-Id: Development discussions about virtio-fs List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: qemu-devel@nongnu.org Cc: Kevin Wolf , Laurent Vivier , qemu-block@nongnu.org, "Michael S. Tsirkin" , Jason Wang , Amit Shah , David Hildenbrand , Raphael Norwitz , virtio-fs@redhat.com, Eric Auger , Hanna Reitz , "Gonglei (Arei)" , Gerd Hoffmann , =?ISO-8859-1?Q?Marc=2DAndr=E9?= Lureau , Paolo Bonzini , Fam Zheng On Dienstag, 9. November 2021 11:56:35 CET Stefan Hajnoczi wrote: > On Thu, Nov 04, 2021 at 03:41:23PM +0100, Christian Schoenebeck wrote: > > On Mittwoch, 3. November 2021 12:33:33 CET Stefan Hajnoczi wrote: > > > On Mon, Nov 01, 2021 at 09:29:26PM +0100, Christian Schoenebeck wrote: > > > > On Donnerstag, 28. Oktober 2021 11:00:48 CET Stefan Hajnoczi wrote: > > > > > On Mon, Oct 25, 2021 at 05:03:25PM +0200, Christian Schoenebeck wrote: > > > > > > On Montag, 25. Oktober 2021 12:30:41 CEST Stefan Hajnoczi wrote: > > > > > > > On Thu, Oct 21, 2021 at 05:39:28PM +0200, Christian Schoenebeck wrote: > > > > > > > > On Freitag, 8. Oktober 2021 18:08:48 CEST Christian Schoenebeck wrote: > > > > > > > > > On Freitag, 8. Oktober 2021 16:24:42 CEST Christian Schoenebeck wrote: > > > > > > > > > > On Freitag, 8. Oktober 2021 09:25:33 CEST Greg Kurz wrote: > > > > > > > > > > > On Thu, 7 Oct 2021 16:42:49 +0100 > > > > > > > > > > > > > > > > > > > > > > Stefan Hajnoczi wrote: > > > > > > > > > > > > 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/c > > > > > > > > > > > > > > > s01/ > > > > > > > > > > > > > > > virt > > > > > > > > > > > > > > > io-v > > > > > > > > > > > > > > > 1.1- > > > > > > > > > > > > > > > cs > > > > > > > > > > > > > > > 01 > > > > > > > > > > > > > > > .html# > > > > > > > > > > > > > > > x1-240006 > > > > > > > > > > > > > > > > > > > > > > > > > > > > Hi Christian, > > > > > > > > > > > > > > > > > > > > > > > > > I took a quick look at the code: > > > > > > > > > > > Hi, > > > > > > > > > > > > > > > > > > > > > > Thanks Stefan for sharing virtio expertise and helping > > > > > > > > > > > Christian > > > > > > > > > > > ! > > > > > > > > > > > > > > > > > > > > > > > > > - 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. > > > > > > > > > > > > > linu > > > > > > > > > > > > > x_os > > > > > > > > > > > > > s@cr > > > > > > > > > > > > > udeb > > > > > > > > > > > > > yt > > > > > > > > > > > > > e. > > > > > > > > > > > > > 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. > > > > > > > > > > > > > > > > > > > > Nobody has reviewed the kernel patches yet. My main > > > > > > > > > > concern > > > > > > > > > > therefore > > > > > > > > > > actually is that the kernel patches are already too > > > > > > > > > > complex, > > > > > > > > > > because > > > > > > > > > > the > > > > > > > > > > current situation is that only Dominique is handling 9p > > > > > > > > > > patches on > > > > > > > > > > kernel > > > > > > > > > > side, and he barely has time for 9p anymore. > > > > > > > > > > > > > > > > > > > > Another reason for me to catch up on reading current > > > > > > > > > > kernel > > > > > > > > > > code > > > > > > > > > > and > > > > > > > > > > stepping in as reviewer of 9p on kernel side ASAP, > > > > > > > > > > independent > > > > > > > > > > of > > > > > > > > > > this > > > > > > > > > > issue. > > > > > > > > > > > > > > > > > > > > As for current kernel patches' complexity: I can certainly > > > > > > > > > > drop > > > > > > > > > > patch > > > > > > > > > > 7 > > > > > > > > > > entirely as it is probably just overkill. Patch 4 is then > > > > > > > > > > the > > > > > > > > > > biggest > > > > > > > > > > chunk, I have to see if I can simplify it, and whether it > > > > > > > > > > would > > > > > > > > > > make > > > > > > > > > > sense to squash with patch 3. > > > > > > > > > > > > > > > > > > > > > > > > - 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); > > > > > > > > > > > > > > > > > > > > > > Note that this is done twice : once for the T message > > > > > > > > > > > (client > > > > > > > > > > > request) > > > > > > > > > > > and > > > > > > > > > > > once for the R message (server answer). The 9p driver > > > > > > > > > > > could > > > > > > > > > > > adjust > > > > > > > > > > > the > > > > > > > > > > > size > > > > > > > > > > > of the T message to what's really needed instead of > > > > > > > > > > > allocating > > > > > > > > > > > the > > > > > > > > > > > full > > > > > > > > > > > msize. R message size is not known though. > > > > > > > > > > > > > > > > > > > > Would it make sense adding a second virtio ring, dedicated > > > > > > > > > > to > > > > > > > > > > server > > > > > > > > > > responses to solve this? IIRC 9p server already calculates > > > > > > > > > > appropriate > > > > > > > > > > exact sizes for each response type. So server could just > > > > > > > > > > push > > > > > > > > > > space > > > > > > > > > > that's > > > > > > > > > > really needed for its responses. > > > > > > > > > > > > > > > > > > > > > > > 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. > > > > > > > > > > > > > > > > > > > > Hu, I didn't even consider vmalloc(). I just tried the > > > > > > > > > > kvmalloc() > > > > > > > > > > wrapper > > > > > > > > > > as a quick & dirty test, but it crashed in the same way as > > > > > > > > > > kmalloc() > > > > > > > > > > with > > > > > > > > > > large msize values immediately on mounting: > > > > > > > > > > > > > > > > > > > > diff --git a/net/9p/client.c b/net/9p/client.c > > > > > > > > > > index a75034fa249b..cfe300a4b6ca 100644 > > > > > > > > > > --- a/net/9p/client.c > > > > > > > > > > +++ b/net/9p/client.c > > > > > > > > > > @@ -227,15 +227,18 @@ static int parse_opts(char *opts, > > > > > > > > > > struct > > > > > > > > > > p9_client > > > > > > > > > > *clnt) > > > > > > > > > > > > > > > > > > > > static int p9_fcall_init(struct p9_client *c, struct > > > > > > > > > > p9_fcall > > > > > > > > > > *fc, > > > > > > > > > > > > > > > > > > > > int alloc_msize) > > > > > > > > > > > > > > > > > > > > { > > > > > > > > > > > > > > > > > > > > - if (likely(c->fcall_cache) && alloc_msize == > > > > > > > > > > c->msize) > > > > > > > > > > { > > > > > > > > > > + //if (likely(c->fcall_cache) && alloc_msize == > > > > > > > > > > c->msize) { > > > > > > > > > > + if (false) { > > > > > > > > > > > > > > > > > > > > fc->sdata = > > > > > > > > > > kmem_cache_alloc(c->fcall_cache, > > > > > > > > > > GFP_NOFS); > > > > > > > > > > fc->cache = c->fcall_cache; > > > > > > > > > > > > > > > > > > > > } else { > > > > > > > > > > > > > > > > > > > > - fc->sdata = kmalloc(alloc_msize, > > > > > > > > > > GFP_NOFS); > > > > > > > > > > + fc->sdata = kvmalloc(alloc_msize, > > > > > > > > > > GFP_NOFS); > > > > > > > > > > > > > > > > > > Ok, GFP_NOFS -> GFP_KERNEL did the trick. > > > > > > > > > > > > > > > > > > Now I get: > > > > > > > > > virtio: bogus descriptor or out of resources > > > > > > > > > > > > > > > > > > So, still some work ahead on both ends. > > > > > > > > > > > > > > > > Few hacks later (only changes on 9p client side) I got this > > > > > > > > running > > > > > > > > stable > > > > > > > > now. The reason for the virtio error above was that kvmalloc() > > > > > > > > returns > > > > > > > > a > > > > > > > > non-logical kernel address for any kvmalloc(>4M), i.e. an > > > > > > > > address > > > > > > > > that > > > > > > > > is > > > > > > > > inaccessible from host side, hence that "bogus descriptor" > > > > > > > > message > > > > > > > > by > > > > > > > > QEMU. > > > > > > > > So I had to split those linear 9p client buffers into sparse > > > > > > > > ones > > > > > > > > (set > > > > > > > > of > > > > > > > > individual pages). > > > > > > > > > > > > > > > > I tested this for some days with various virtio transmission > > > > > > > > sizes > > > > > > > > and > > > > > > > > it > > > > > > > > works as expected up to 128 MB (more precisely: 128 MB read > > > > > > > > space > > > > > > > > + > > > > > > > > 128 MB > > > > > > > > write space per virtio round trip message). > > > > > > > > > > > > > > > > I did not encounter a show stopper for large virtio > > > > > > > > transmission > > > > > > > > sizes > > > > > > > > (4 MB ... 128 MB) on virtio level, neither as a result of > > > > > > > > testing, > > > > > > > > nor > > > > > > > > after reviewing the existing code. > > > > > > > > > > > > > > > > About IOV_MAX: that's apparently not an issue on virtio level. > > > > > > > > Most of > > > > > > > > the > > > > > > > > iovec code, both on Linux kernel side and on QEMU side do not > > > > > > > > have > > > > > > > > this > > > > > > > > limitation. It is apparently however indeed a limitation for > > > > > > > > userland > > > > > > > > apps > > > > > > > > calling the Linux kernel's syscalls yet. > > > > > > > > > > > > > > > > Stefan, as it stands now, I am even more convinced that the > > > > > > > > upper > > > > > > > > virtio > > > > > > > > transmission size limit should not be squeezed into the queue > > > > > > > > size > > > > > > > > argument of virtio_add_queue(). Not because of the previous > > > > > > > > argument > > > > > > > > that > > > > > > > > it would waste space (~1MB), but rather because they are two > > > > > > > > different > > > > > > > > things. To outline this, just a quick recap of what happens > > > > > > > > exactly > > > > > > > > when > > > > > > > > a bulk message is pushed over the virtio wire (assuming virtio > > > > > > > > "split" > > > > > > > > layout here): > > > > > > > > > > > > > > > > ---------- [recap-start] ---------- > > > > > > > > > > > > > > > > For each bulk message sent guest <-> host, exactly *one* of > > > > > > > > the > > > > > > > > pre-allocated descriptors is taken and placed (subsequently) > > > > > > > > into > > > > > > > > exactly > > > > > > > > *one* position of the two available/used ring buffers. The > > > > > > > > actual > > > > > > > > descriptor table though, containing all the DMA addresses of > > > > > > > > the > > > > > > > > message > > > > > > > > bulk data, is allocated just in time for each round trip > > > > > > > > message. > > > > > > > > Say, > > > > > > > > it > > > > > > > > is the first message sent, it yields in the following > > > > > > > > structure: > > > > > > > > > > > > > > > > Ring Buffer Descriptor Table Bulk Data Pages > > > > > > > > > > > > > > > > +-+ +-+ +-----------------+ > > > > > > > > > > > > > > > > |D|------------->|d|---------->| Bulk data block | > > > > > > > > > > > > > > > > +-+ |d|--------+ +-----------------+ > > > > > > > > > > > > > > > > | | |d|------+ | > > > > > > > > > > > > > > > > +-+ . | | +-----------------+ > > > > > > > > > > > > > > > > | | . | +->| Bulk data block | > > > > > > > > > > > > > > > > . . | +-----------------+ > > > > > > > > . |d|-+ | > > > > > > > > . +-+ | | +-----------------+ > > > > > > > > > > > > > > > > | | | +--->| Bulk data block | > > > > > > > > > > > > > > > > +-+ | +-----------------+ > > > > > > > > > > > > > > > > | | | . > > > > > > > > > > > > > > > > +-+ | . > > > > > > > > > > > > > > > > | . > > > > > > > > | > > > > > > > > | +-----------------+ > > > > > > > > > > > > > > > > +-------->| Bulk data block | > > > > > > > > > > > > > > > > +-----------------+ > > > > > > > > > > > > > > > > Legend: > > > > > > > > D: pre-allocated descriptor > > > > > > > > d: just in time allocated descriptor > > > > > > > > -->: memory pointer (DMA) > > > > > > > > > > > > > > > > The bulk data blocks are allocated by the respective device > > > > > > > > driver > > > > > > > > above > > > > > > > > virtio subsystem level (guest side). > > > > > > > > > > > > > > > > There are exactly as many descriptors pre-allocated (D) as the > > > > > > > > size of > > > > > > > > a > > > > > > > > ring buffer. > > > > > > > > > > > > > > > > A "descriptor" is more or less just a chainable DMA memory > > > > > > > > pointer; > > > > > > > > defined > > > > > > > > as: > > > > > > > > > > > > > > > > /* Virtio ring descriptors: 16 bytes. These can chain > > > > > > > > together > > > > > > > > via > > > > > > > > "next". */ struct vring_desc { > > > > > > > > > > > > > > > > /* Address (guest-physical). */ > > > > > > > > __virtio64 addr; > > > > > > > > /* Length. */ > > > > > > > > __virtio32 len; > > > > > > > > /* The flags as indicated above. */ > > > > > > > > __virtio16 flags; > > > > > > > > /* We chain unused descriptors via this, too */ > > > > > > > > __virtio16 next; > > > > > > > > > > > > > > > > }; > > > > > > > > > > > > > > > > There are 2 ring buffers; the "available" ring buffer is for > > > > > > > > sending a > > > > > > > > message guest->host (which will transmit DMA addresses of > > > > > > > > guest > > > > > > > > allocated > > > > > > > > bulk data blocks that are used for data sent to device, and > > > > > > > > separate > > > > > > > > guest allocated bulk data blocks that will be used by host > > > > > > > > side to > > > > > > > > place > > > > > > > > its response bulk data), and the "used" ring buffer is for > > > > > > > > sending > > > > > > > > host->guest to let guest know about host's response and that > > > > > > > > it > > > > > > > > could > > > > > > > > now > > > > > > > > safely consume and then deallocate the bulk data blocks > > > > > > > > subsequently. > > > > > > > > > > > > > > > > ---------- [recap-end] ---------- > > > > > > > > > > > > > > > > So the "queue size" actually defines the ringbuffer size. It > > > > > > > > does > > > > > > > > not > > > > > > > > define the maximum amount of descriptors. The "queue size" > > > > > > > > rather > > > > > > > > defines > > > > > > > > how many pending messages can be pushed into either one > > > > > > > > ringbuffer > > > > > > > > before > > > > > > > > the other side would need to wait until the counter side would > > > > > > > > step up > > > > > > > > (i.e. ring buffer full). > > > > > > > > > > > > > > > > The maximum amount of descriptors (what VIRTQUEUE_MAX_SIZE > > > > > > > > actually > > > > > > > > is) > > > > > > > > OTOH defines the max. bulk data size that could be transmitted > > > > > > > > with > > > > > > > > each > > > > > > > > virtio round trip message. > > > > > > > > > > > > > > > > And in fact, 9p currently handles the virtio "queue size" as > > > > > > > > directly > > > > > > > > associative with its maximum amount of active 9p requests the > > > > > > > > server > > > > > > > > could > > > > > > > > > > > > > > > > handle simultaniously: > > > > > > > > hw/9pfs/9p.h:#define MAX_REQ 128 > > > > > > > > hw/9pfs/9p.h: V9fsPDU pdus[MAX_REQ]; > > > > > > > > hw/9pfs/virtio-9p-device.c: v->vq = > > > > > > > > virtio_add_queue(vdev, > > > > > > > > MAX_REQ, > > > > > > > > > > > > > > > > handle_9p_output); > > > > > > > > > > > > > > > > So if I would change it like this, just for the purpose to > > > > > > > > increase > > > > > > > > the > > > > > > > > max. virtio transmission size: > > > > > > > > > > > > > > > > --- a/hw/9pfs/virtio-9p-device.c > > > > > > > > +++ b/hw/9pfs/virtio-9p-device.c > > > > > > > > @@ -218,7 +218,7 @@ static void > > > > > > > > virtio_9p_device_realize(DeviceState > > > > > > > > *dev, > > > > > > > > Error **errp)> > > > > > > > > > > > > > > > > v->config_size = sizeof(struct virtio_9p_config) + > > > > > > > > strlen(s->fsconf.tag); > > > > > > > > virtio_init(vdev, "virtio-9p", VIRTIO_ID_9P, > > > > > > > > v->config_size, > > > > > > > > > > > > > > > > VIRTQUEUE_MAX_SIZE); > > > > > > > > > > > > > > > > - v->vq = virtio_add_queue(vdev, MAX_REQ, > > > > > > > > handle_9p_output); > > > > > > > > + v->vq = virtio_add_queue(vdev, 32*1024, > > > > > > > > handle_9p_output); > > > > > > > > > > > > > > > > } > > > > > > > > > > > > > > > > Then it would require additional synchronization code on both > > > > > > > > ends > > > > > > > > and > > > > > > > > therefore unnecessary complexity, because it would now be > > > > > > > > possible > > > > > > > > that > > > > > > > > more requests are pushed into the ringbuffer than server could > > > > > > > > handle. > > > > > > > > > > > > > > > > There is one potential issue though that probably did justify > > > > > > > > the > > > > > > > > "don't > > > > > > > > exceed the queue size" rule: > > > > > > > > > > > > > > > > ATM the descriptor table is allocated (just in time) as *one* > > > > > > > > continuous > > > > > > > > buffer via kmalloc_array(): > > > > > > > > https://github.com/torvalds/linux/blob/2f111a6fd5b5297b4e92f53 > > > > > > > > 798c > > > > > > > > a086 > > > > > > > > f7c7 > > > > > > > > d33a4/drivers/virtio/virtio_ring.c#L440 > > > > > > > > > > > > > > > > So assuming transmission size of 2 * 128 MB that > > > > > > > > kmalloc_array() > > > > > > > > call > > > > > > > > would > > > > > > > > yield in kmalloc(1M) and the latter might fail if guest had > > > > > > > > highly > > > > > > > > fragmented physical memory. For such kind of error case there > > > > > > > > is > > > > > > > > currently a fallback path in virtqueue_add_split() that would > > > > > > > > then > > > > > > > > use > > > > > > > > the required amount of pre-allocated descriptors instead: > > > > > > > > https://github.com/torvalds/linux/blob/2f111a6fd5b5297b4e92f53 > > > > > > > > 798c > > > > > > > > a086 > > > > > > > > f7c7 > > > > > > > > d33a4/drivers/virtio/virtio_ring.c#L525 > > > > > > > > > > > > > > > > That fallback recovery path would no longer be viable if the > > > > > > > > queue > > > > > > > > size > > > > > > > > was > > > > > > > > exceeded. There would be alternatives though, e.g. by allowing > > > > > > > > to > > > > > > > > chain > > > > > > > > indirect descriptor tables (currently prohibited by the virtio > > > > > > > > specs). > > > > > > > > > > > > > > Making the maximum number of descriptors independent of the > > > > > > > queue > > > > > > > size > > > > > > > requires a change to the VIRTIO spec since the two values are > > > > > > > currently > > > > > > > explicitly tied together by the spec. > > > > > > > > > > > > Yes, that's what the virtio specs say. But they don't say why, nor > > > > > > did > > > > > > I > > > > > > hear a reason in this dicussion. > > > > > > > > > > > > That's why I invested time reviewing current virtio implementation > > > > > > and > > > > > > specs, as well as actually testing exceeding that limit. And as I > > > > > > outlined in detail in my previous email, I only found one > > > > > > theoretical > > > > > > issue that could be addressed though. > > > > > > > > > > I agree that there is a limitation in the VIRTIO spec, but violating > > > > > the > > > > > spec isn't an acceptable solution: > > > > > > > > > > 1. QEMU and Linux aren't the only components that implement VIRTIO. > > > > > You > > > > > > > > > > cannot make assumptions about their implementations because it > > > > > may > > > > > break spec-compliant implementations that you haven't looked at. > > > > > > > > > > Your patches weren't able to increase Queue Size because some > > > > > device > > > > > implementations break when descriptor chains are too long. This > > > > > shows > > > > > there is a practical issue even in QEMU. > > > > > > > > > > 2. The specific spec violation that we discussed creates the problem > > > > > > > > > > that drivers can no longer determine the maximum description > > > > > chain > > > > > length. This in turn will lead to more implementation-specific > > > > > assumptions being baked into drivers and cause problems with > > > > > interoperability and future changes. > > > > > > > > > > The spec needs to be extended instead. I included an idea for how to > > > > > do > > > > > that below. > > > > > > > > Sure, I just wanted to see if there was a non-neglectable "hard" show > > > > stopper per se that I probably haven't seen yet. I have not questioned > > > > aiming a clean solution. > > > > > > > > Thanks for the clarification! > > > > > > > > > > > Before doing that, are there benchmark results showing that 1 MB > > > > > > > vs > > > > > > > 128 > > > > > > > MB produces a performance improvement? I'm asking because if > > > > > > > performance > > > > > > > with 1 MB is good then you can probably do that without having > > > > > > > to > > > > > > > change > > > > > > > VIRTIO and also because it's counter-intuitive that 9p needs 128 > > > > > > > MB > > > > > > > for > > > > > > > good performance when it's ultimately implemented on top of disk > > > > > > > and > > > > > > > network I/O that have lower size limits. > > > > > > > > > > > > First some numbers, linear reading a 12 GB file: > > > > > > > > > > > > msize average notes > > > > > > > > > > > > 8 kB 52.0 MB/s default msize of Linux kernel > > > > > 128 kB 624.8 MB/s default msize of Linux kernel >=v5.15 > > > > > > 512 kB 1961 MB/s current max. msize with any Linux kernel > > > > > > <=v5.15 > > > > > > 1 MB 2551 MB/s this msize would already violate virtio > > > > > > specs > > > > > > 2 MB 2521 MB/s this msize would already violate virtio > > > > > > specs > > > > > > 4 MB 2628 MB/s planned max. msize of my current kernel > > > > > > patches > > > > > > [1] > > > > > > > > > > How many descriptors are used? 4 MB can be covered by a single > > > > > descriptor if the data is physically contiguous in memory, so this > > > > > data > > > > > doesn't demonstrate a need for more descriptors. > > > > > > > > No, in the last couple years there was apparently no kernel version > > > > that > > > > used just one descriptor, nor did my benchmarked version. Even though > > > > the > > > > Linux 9p client uses (yet) simple linear buffers (contiguous physical > > > > memory) on 9p client level, these are however split into PAGE_SIZE > > > > chunks > > > > by function pack_sg_list() [1] before being fed to virtio level: > > > > > > > > static unsigned int rest_of_page(void *data) > > > > { > > > > > > > > return PAGE_SIZE - offset_in_page(data); > > > > > > > > } > > > > ... > > > > static int pack_sg_list(struct scatterlist *sg, int start, > > > > > > > > int limit, char *data, int count) > > > > > > > > { > > > > > > > > int s; > > > > int index = start; > > > > > > > > while (count) { > > > > > > > > s = rest_of_page(data); > > > > ... > > > > sg_set_buf(&sg[index++], data, s); > > > > count -= s; > > > > data += s; > > > > > > > > } > > > > ... > > > > > > > > } > > > > > > > > [1] > > > > https://github.com/torvalds/linux/blob/19901165d90fdca1e57c9baa0d5b4c6 > > > > 3d1 > > > > 5c476a/net/9p/trans_virtio.c#L171 > > > > > > > > So when sending 4MB over virtio wire, it would yield in 1k descriptors > > > > ATM. > > > > > > > > I have wondered about this before, but did not question it, because > > > > due to > > > > the cross-platform nature I couldn't say for certain whether that's > > > > probably needed somewhere. I mean for the case virtio-PCI I know for > > > > sure > > > > that one descriptor (i.e. >PAGE_SIZE) would be fine, but I don't know > > > > if > > > > that applies to all buses and architectures. > > > > > > VIRTIO does not limit descriptor the descriptor len field to PAGE_SIZE, > > > so I don't think there is a limit at the VIRTIO level. > > > > So you are viewing this purely from virtio specs PoV: in the sense, if it > > is not prohibited by the virtio specs, then it should work. Maybe. > > Limitations must be specified either in the 9P protocol or the VIRTIO > specification. Drivers and devices will not be able to operate correctly > if there are limitations that aren't covered by the specs. > > Do you have something in mind that isn't covered by the specs? Not sure whether that's something that should be specified by the virtio specs, probably not. I simply do not know if there was any bus or architecture that would have a limitation for max. size for a memory block passed per one DMA address. > > > If this function coalesces adjacent pages then the descriptor chain > > > length issues could be reduced. > > > > > > > > > But again, this is not just about performance. My conclusion as > > > > > > described > > > > > > in my previous email is that virtio currently squeezes > > > > > > > > > > > > "max. simultanious amount of bulk messages" > > > > > > > > > > > > vs. > > > > > > > > > > > > "max. bulk data transmission size per bulk messaage" > > > > > > > > > > > > into the same configuration parameter, which is IMO inappropriate > > > > > > and > > > > > > hence > > > > > > splitting them into 2 separate parameters when creating a queue > > > > > > makes > > > > > > sense, independent of the performance benchmarks. > > > > > > > > > > > > [1] > > > > > > https://lore.kernel.org/netdev/cover.1632327421.git.linux_oss@crud > > > > > > ebyt > > > > > > e.c > > > > > > om/ > > > > > > > > > > Some devices effectively already have this because the device > > > > > advertises > > > > > a maximum number of descriptors via device-specific mechanisms like > > > > > the > > > > > struct virtio_blk_config seg_max field. But today these fields can > > > > > only > > > > > reduce the maximum descriptor chain length because the spec still > > > > > limits > > > > > the length to Queue Size. > > > > > > > > > > We can build on this approach to raise the length above Queue Size. > > > > > This > > > > > approach has the advantage that the maximum number of segments isn't > > > > > per > > > > > device or per virtqueue, it's fine-grained. If the device supports > > > > > two > > > > > requests types then different max descriptor chain limits could be > > > > > given > > > > > for them by introducing two separate configuration space fields. > > > > > > > > > > Here are the corresponding spec changes: > > > > > > > > > > 1. A new feature bit called VIRTIO_RING_F_LARGE_INDIRECT_DESC is > > > > > added > > > > > > > > > > to indicate that indirect descriptor table size and maximum > > > > > descriptor chain length are not limited by Queue Size value. > > > > > (Maybe > > > > > there still needs to be a limit like 2^15?) > > > > > > > > Sounds good to me! > > > > > > > > AFAIK it is effectively limited to 2^16 because of vring_desc->next: > > > > > > > > /* Virtio ring descriptors: 16 bytes. These can chain together via > > > > "next". */ struct vring_desc { > > > > > > > > /* Address (guest-physical). */ > > > > __virtio64 addr; > > > > /* Length. */ > > > > __virtio32 len; > > > > /* The flags as indicated above. */ > > > > __virtio16 flags; > > > > /* We chain unused descriptors via this, too */ > > > > __virtio16 next; > > > > > > > > }; > > > > > > Yes, Split Virtqueues have a fundamental limit on indirect table size > > > due to the "next" field. Packed Virtqueue descriptors don't have a > > > "next" field so descriptor chains could be longer in theory (currently > > > forbidden by the spec). > > > > > > > > One thing that's messy is that we've been discussing the maximum > > > > > descriptor chain length but 9p has the "msize" concept, which isn't > > > > > aware of contiguous memory. It may be necessary to extend the 9p > > > > > driver > > > > > code to size requests not just according to their length in bytes > > > > > but > > > > > also according to the descriptor chain length. That's how the Linux > > > > > block layer deals with queue limits (struct queue_limits > > > > > max_segments vs > > > > > max_hw_sectors). > > > > > > > > Hmm, can't follow on that one. For what should that be needed in case > > > > of > > > > 9p? My plan was to limit msize by 9p client simply at session start to > > > > whatever is the max. amount virtio descriptors supported by host and > > > > using PAGE_SIZE as size per descriptor, because that's what 9p client > > > > actually does ATM (see above). So you think that should be changed to > > > > e.g. just one descriptor for 4MB, right? > > > > > > Limiting msize to the 9p transport device's maximum number of > > > descriptors is conservative (i.e. 128 descriptors = 512 KB msize) > > > because it doesn't take advantage of contiguous memory. I suggest > > > leaving msize alone, adding a separate limit at which requests are split > > > according to the maximum descriptor chain length, and tweaking > > > pack_sg_list() to coalesce adjacent pages. > > > > > > That way msize can be large without necessarily using lots of > > > descriptors (depending on the memory layout). > > > > That was actually a tempting solution. Because it would neither require > > changes to the virtio specs (at least for a while) and it would also work > > with older QEMU versions. And for that pack_sg_list() portion of the code > > it would work well and easy as the buffer passed to pack_sg_list() is > > contiguous already. > > > > However I just realized for the zero-copy version of the code that would > > be > > more tricky. The ZC version already uses individual pages (struct page, > > hence PAGE_SIZE each) which are pinned, i.e. it uses pack_sg_list_p() [1] > > in combination with p9_get_mapped_pages() [2] > > > > [1] > > https://github.com/torvalds/linux/blob/7ddb58cb0ecae8e8b6181d736a87667cc9 > > ab8389/net/9p/trans_virtio.c#L218 [2] > > https://github.com/torvalds/linux/blob/7ddb58cb0ecae8e8b6181d736a87667cc9 > > ab8389/net/9p/trans_virtio.c#L309 > > > > So that would require much more work and code trying to sort and coalesce > > individual pages to contiguous physical memory for the sake of reducing > > virtio descriptors. And there is no guarantee that this is even possible. > > The kernel may simply return a non-contiguous set of pages which would > > eventually end up exceeding the virtio descriptor limit again. > > Order must be preserved so pages cannot be sorted by physical address. > How about simply coalescing when pages are adjacent? It would help, but not solve the issue we are talking about here: if 99% of the cases could successfully merge descriptors to stay below the descriptor count limit, but in 1% of the cases it could not, then this still construes a severe runtime issue that could trigger at any time. > > So looks like it was probably still easier and realistic to just add > > virtio > > capabilities for now for allowing to exceed current descriptor limit. > > I'm still not sure why virtio-net, virtio-blk, virtio-fs, etc perform > fine under today's limits while virtio-9p needs a much higher limit to > achieve good performance. Maybe there is an issue in a layer above the > vring that's causing the virtio-9p performance you've observed? Are you referring to (somewhat) recent benchmarks when saying those would all still perform fine today? Vivek was running detailed benchmarks for virtiofs vs. 9p: https://lists.gnu.org/archive/html/qemu-devel/2020-12/msg02704.html For the virtio aspect discussed here, only the benchmark configurations without cache are relevant (9p-none, vtfs-none) and under this aspect the situation seems to be quite similar between 9p and virtio-fs. You'll also note once DAX is enabled (vtfs-none-dax) that apparently boosts virtio-fs performance significantly, which however seems to corelate to numbers when I am running 9p with msize > 300k. Note: Vivek was presumably running 9p effecively with msize=300k, as this was the kernel limitation at that time. To bring things into relation: there are known performance aspects in 9p that can be improved, yes, both on Linux kernel side and on 9p server side in QEMU. For instance 9p server uses coroutines [1] and currently dispatches between worker thread(s) and main thread too often per request (partly addressed already [2], but still WIP), which accumulates to overall latency. But Vivek was actually using a 9p patch here which disabled coroutines entirely, which suggests that the virtio transmission size limit still represents a bottleneck. [1] https://wiki.qemu.org/Documentation/9p#Coroutines [2] https://wiki.qemu.org/Documentation/9p#Implementation_Plans Best regards, Christian Schoenebeck