Re: [Qemu-devel] Why I advise against using ivshmem

[Vincent JARDIN]

(+merging with Paolo's email because of overlaps)

> > see inline (I am not on all mailing list, please, keep the cc list).

> > > 1. ivshmem code needs work, but has no maintainer
> > See David's contributions:
> >    http://patchwork.ozlabs.org/patch/358750/
>
> We're grateful for David's patch for qemu-char.c, but this isn't ivshmem
> maintenance, yet.

others can come (doc), see below.

> > > 2. There is no libvirt support
> >
> > One can use qemu without libvivrt.
>
> You asked me for my reasons for disliking ivshmem.  This is one.
>
> Sure, I can drink my water through a straw while standing on one foot,
> but that doesn't mean I have to like it.  And me not liking it doesn't
> mean the next guy shouldn't like it.  To each their own.

I like using qemu without libvirt, libvirt is not part of qemu.
Let's avoid trolling about it ;)

> Back when we accepted ivshmem, the out-of-tree parts it needs were well
> below the "community & packaged" bar.  But folks interested in it talked
> to us, and the fact that it's in shows that QEMU maintainers decided
> what they had then was enough.
>
> Unfortunately, we now have considerably less: Nahanni appears to be
> dead.

agree and to bad it is dead. We should let Nahanni dead since ivshmem is 
a QEMU topic now, see below. Does it make sense?

> An apparently dead git repository you can study is not enough.  The fact
> that you hold an improved reimplementation privately is immaterial.  So
> is the (plausible) claim that others could also create a
> reimplementation.

Got the point. What's about a patch to 
docs/specs/ivshmem_device_spec.txt that improves it?

I can make qemu's ivshmem better:
  - keep explaining memnic for instance,
  - explain how to write other ivshmem.

does it help?

> > > 4. Out-of-tree kernel uio driver required
> >
> > No, it is optional.
>
> Good to know.  Would you be willing to send a patch to
> ivshmem_device_spec.txt clarifying that?

got the point, yes,

> > > * Get all the required parts outside QEMU packaged in major distros, or
> > >     absorbed into QEMU
> >
> > Redhat did disable it. why? it is there in QEMU.
>
> Up to now, I've been wearing my QEMU hat.  Let me exchange it for my Red
> one for a bit.
>
> We (Red Hat) don't just package & ship metric tons of random free
> software.  We package & ship useful free software we can support for
> many, many years.
>
> Sometimes, we find that we have to focus serious development resources
> on making something useful supportable (Paolo mentioned qcow2).  We
> obviously can't focus on everything, though.

Good open technology should rule. ivshmem has use cases. And I go agree 
with you, it is like the phoenix, it has to be re-explained/documented 
to be back to life. I was not aware that the QEMU community was missing 
ivshmem contributors (my bad I did not check MAINTAINERS).

> Anyway, ivshmem didn't make the cut for RHEL-7.0.  Sorry if that
> inconveniences you.  To get it into RHEL, you need to show it's both
> useful and supportable.  Building a community around it would go a long
> way towards that.

understood.

> If you want to discuss this in more detail with us, you may want to try
> communication channels provided by your RHEL subscription in addition to
> the QEMU development mailing list.  Don't be shy, you're paying for it!

done. I was focusing on DPDK.org and ignorant of QEMU's status, thinking 
Redhat was covering it. How to know which part of an opensource software 
are and are not included into Redhat. Sales are ignorant about it ;). 
Redhat randomly disables some files at compilation (for some good 
reasons I guess, but not public rationals or I am missing something).

Feel free to open this PR to anyone:
  https://bugzilla.redhat.com/show_bug.cgi?id=1088332

> > > In short, create a viable community around ivshmem, either within the
> > > QEMU community, or separately but cooperating.
> >
> > At least, DPDK.org community is a community using it.
>
> Using something isn't the same as maintaining something.  But it's a
> necessary first step.

understood, after David's patch, documentation will come.

(now Paolo's email since there were some overlaps)

> Markus especially referred to parts *outside* QEMU: the server, the
> uio driver, etc.  These out-of-tree, non-packaged parts of ivshmem
> are one of the reasons why Red Hat has disabled ivshmem in RHEL7.

You made the right choices, these out-of-tree packages are not required. 
You can use QEMU's ivshmem without any of the out-of-tree packages. The 
out-of-tree packages are just some examples of using ivshmem.

> He also listed many others.  Basically for parts of QEMU that are not
> of high quality, we either fix them (this is for example what we did
> for qcow2) or disable them.  Not just ivshmem suffered this fate, for
> example many network cards, sound cards, SCSI storage adapters.

I and David (cc) are working on making it better based on the issues 
that are found.

> Now, vhost-user is in the process of being merged for 2.1.  Compared 
to the DPDK solution:

now, you cannot compare vhost-user to DPDK/ivshmem; both should exsit 
because they have different scope and use cases. It is like comparing 
two different(A) models of IPC:
  - vhost-user -> networking use case specific
  - ivshmem -> framework to be generic to have shared memory for many 
use cases (HPC, in-memory-database, a network too like memnic).

Later one, some news services will be needed for shared memory. virtio 
will come in picture (see VIRTIO_F_RING_SHMEM_ADDR's threads). 
Currently, ivhsmem is the only "stable" option since there remains many 
unsolved issues with virtio and shared memory.

> * it doesn't require hugetlbfs (which only enabled shared memory by
> chance in older QEMU releases, that was never documented)

ivhsmem does not require hugetlbfs. It is optional.

> * it doesn't require ivshmem (it does require shared memory, which
> will also be added to 2.1)

somehow I agree: we need both models: vhost-user and ivshmem because of 
the previous (A) comments.

> * it doesn't require the kernel driver from the DPDK sample

ivhsmem does not require DPDK kernel driver. see memnic's PMD:
  http://dpdk.org/browse/memnic/tree/pmd/pmd_memnic.c

> * it is not just shared memory, but also defines an interface to use
> it (another of Markus's points)

agreed Paolo: but you short narrow it for networking use cases only. 
Shared memory à la ivshmem provides other features (see (A) again).

>
> vhost-user is superior, and it is superior because it has been
> designed
> from the get-go through cooperation of all interested parties (namely
> QEMU and snabbswitch).

It is not an argument. vhost-user is a specific case.

Best regards,
  Vincent