Re: [PATCH v2 1/3] virtio: turn VIRTQUEUE_MAX_SIZE into a variable

[Christian Schoenebeck <qemu_oss@crudebyte.com>]

On Dienstag, 5. Oktober 2021 17:10:40 CEST Stefan Hajnoczi wrote:
> On Tue, Oct 05, 2021 at 03:15:26PM +0200, Christian Schoenebeck wrote:
> > On Dienstag, 5. Oktober 2021 14:45:56 CEST Stefan Hajnoczi wrote:
> > > On Mon, Oct 04, 2021 at 09:38:04PM +0200, Christian Schoenebeck wrote:
> > > > Refactor VIRTQUEUE_MAX_SIZE to effectively become a runtime
> > > > variable per virtio user.
> > > 
> > > virtio user == virtio device model?
> > 
> > Yes
> > 
> > > > Reasons:
> > > > 
> > > > (1) VIRTQUEUE_MAX_SIZE should reflect the absolute theoretical
> > > > 
> > > >     maximum queue size possible. Which is actually the maximum
> > > >     queue size allowed by the virtio protocol. The appropriate
> > > >     value for VIRTQUEUE_MAX_SIZE would therefore be 32768:
> > > >     
> > > >     https://docs.oasis-open.org/virtio/virtio/v1.1/cs01/virtio-v1.1-cs
> > > >     01.h
> > > >     tml#x1-240006
> > > >     
> > > >     Apparently VIRTQUEUE_MAX_SIZE was instead defined with a
> > > >     more or less arbitrary value of 1024 in the past, which
> > > >     limits the maximum transfer size with virtio to 4M
> > > >     (more precise: 1024 * PAGE_SIZE, with the latter typically
> > > >     being 4k).
> > > 
> > > Being equal to IOV_MAX is a likely reason. Buffers with more iovecs than
> > > that cannot be passed to host system calls (sendmsg(2), pwritev(2),
> > > etc).
> > 
> > Yes, that's use case dependent. Hence the solution to opt-in if it is
> > desired and feasible.
> > 
> > > > (2) Additionally the current value of 1024 poses a hidden limit,
> > > > 
> > > >     invisible to guest, which causes a system hang with the
> > > >     following QEMU error if guest tries to exceed it:
> > > >     
> > > >     virtio: too many write descriptors in indirect table
> > > 
> > > I don't understand this point. 2.6.5 The Virtqueue Descriptor Table 
says:
> > >   The number of descriptors in the table is defined by the queue size
> > >   for
> > > 
> > > this virtqueue: this is the maximum possible descriptor chain length.
> > > 
> > > and 2.6.5.3.1 Driver Requirements: Indirect Descriptors says:
> > >   A driver MUST NOT create a descriptor chain longer than the Queue Size
> > >   of
> > > 
> > > the device.
> > > 
> > > Do you mean a broken/malicious guest driver that is violating the spec?
> > > That's not a hidden limit, it's defined by the spec.
> > 
> > https://lists.gnu.org/archive/html/qemu-devel/2021-10/msg00781.html
> > https://lists.gnu.org/archive/html/qemu-devel/2021-10/msg00788.html
> > 
> > You can already go beyond that queue size at runtime with the indirection
> > table. The only actual limit is the currently hard coded value of 1k
> > pages.
> > Hence the suggestion to turn that into a variable.
> 
> Exceeding Queue Size is a VIRTIO spec violation. Drivers that operate
> outsided the spec do so at their own risk. They may not be compatible
> with all device implementations.

Yes, I am ware about that. And still, this practice is already done, which 
apparently is not limited to 9pfs.

> The limit is not hidden, it's Queue Size as defined by the spec :).
> 
> If you have a driver that is exceeding the limit, then please fix the
> driver.

I absolutely understand your position, but I hope you also understand that 
this violation of the specs is a theoretical issue, it is not a real-life 
problem right now, and due to lack of man power unfortunately I have to 
prioritize real-life problems over theoretical ones ATM. Keep in mind that 
right now I am the only person working on 9pfs actively, I do this voluntarily 
whenever I find a free time slice, and I am not paid for it either.

I don't see any reasonable way with reasonable effort to do what you are 
asking for here in 9pfs, and Greg may correct me here if I am saying anything 
wrong. If you are seeing any specific real-life issue here, then please tell 
me which one, otherwise I have to postpone that "specs violation" issue.

There is still a long list of real problems that I need to hunt down in 9pfs, 
afterwards I can continue with theoretical ones if you want, but right now I 
simply can't, sorry.

> > > > (3) Unfortunately not all virtio users in QEMU would currently
> > > > 
> > > >     work correctly with the new value of 32768.
> > > > 
> > > > So let's turn this hard coded global value into a runtime
> > > > variable as a first step in this commit, configurable for each
> > > > virtio user by passing a corresponding value with virtio_init()
> > > > call.
> > > 
> > > virtio_add_queue() already has an int queue_size argument, why isn't
> > > that enough to deal with the maximum queue size? There's probably a good
> > > reason for it, but please include it in the commit description.
> > 
> > [...]
> > 
> > > Can you make this value per-vq instead of per-vdev since virtqueues can
> > > have different queue sizes?
> > > 
> > > The same applies to the rest of this patch. Anything using
> > > vdev->queue_max_size should probably use vq->vring.num instead.
> > 
> > I would like to avoid that and keep it per device. The maximum size stored
> > there is the maximum size supported by virtio user (or vortio device
> > model,
> > however you want to call it). So that's really a limit per device, not per
> > queue, as no queue of the device would ever exceed that limit.
> > 
> > Plus a lot more code would need to be refactored, which I think is
> > unnecessary.
> 
> I'm against a per-device limit because it's a concept that cannot
> accurately describe reality. Some devices have multiple classes of

It describes current reality, because VIRTQUEUE_MAX_SIZE obviously is not per 
queue either ATM, and nobody ever cared.

All this series does, is allowing to override that currently project-wide 
compile-time constant to a per-driver-model compile-time constant. Which makes 
sense, because that's what it is: some drivers could cope with any transfer 
size, and some drivers are constrained to a certain maximum application 
specific transfer size (e.g. IOV_MAX).

> virtqueues and they are sized differently, so a per-device limit is
> insufficient. virtio-net has separate rx_queue_size and tx_queue_size
> parameters (plus a control vq hardcoded to 64 descriptors).

I simply find this overkill. This value semantically means "my driver model 
supports at any time and at any coincidence at the very most x * PAGE_SIZE = 
max_transfer_size". Do you see any driver that might want a more fine graded 
control over this?

As far as I can see, no other driver maintainer even seems to care to 
transition to 32k. So I simply doubt that anybody would even want a more 
fained graded control over this in practice, but anyway ...

> The specification already gives us Queue Size (vring.num in QEMU). The
> variable exists in QEMU and just needs to be used.
> 
> If per-vq limits require a lot of work, please describe why. I think
> replacing the variable from this patch with virtio_queue_get_num()
> should be fairly straightforward, but maybe I'm missing something? (If
> you prefer VirtQueue *vq instead of the index-based
> virtio_queue_get_num() API, you can introduce a virtqueue_get_num()
> API.)
> 
> Stefan

... I leave that up to Michael or whoever might be in charge to decide. I 
still find this overkill, but I will adapt this to whatever the decision 
eventually will be in v3.

But then please tell me the precise representation that you find appropriate, 
i.e. whether you want a new function for that, or rather an additional 
argument to virtio_add_queue(). Your call.

Best regards,
Christian Schoenebeck