From: Xuewei Niu <niuxuewei97@gmail.com>
To: sgarzare@redhat.com
Cc: fupan.lfp@antgroup.com, mst@redhat.com,
niuxuewei.nxw@antgroup.com, niuxuewei97@gmail.com,
parav@nvidia.com, stefanha@redhat.com,
virtio-comment@lists.linux.dev
Subject: Re: [PATCH v6 RESEND] virtio-vsock: Add support for multi devices
Date: Thu, 10 Apr 2025 11:05:47 +0800 [thread overview]
Message-ID: <20250410030547.3108952-1-niuxuewei.nxw@antgroup.com> (raw)
In-Reply-To: <CAGxU2F4EpyYg2z5RKq9C-hTDK3Cb06wKJzB_pF_NH_XipAYw=g@mail.gmail.com>
> On Wed, 9 Apr 2025 at 08:56, Xuewei Niu <niuxuewei97@gmail.com> wrote:
> [...]
> > >
> > > The idea is to inform the guest which addresses are reachable by the
> > > device, so the guest can easily decide which device to use. I'm
> > > talking about the destination, so CID_HOST(2), CID_HYPERVISOS(0) or a
> > > sibling VM (CID >=3).
> > >
> > > >
> > > > We have had the guest_cid field in the config space. The guest knows all
> > > > devices present in the VM.
> > >
> > > Okay, but how can the guest figure out from this information which
> > > device to use to talk to the hypervisor or an application in the host?
> > >
> > > >
> > > > If the app tries to bind a random CID, it will fail since the driver can't
> > > > find the device by the CID.
> > >
> > > I'm not talking about the source CID on which to do bind() (which I
> > > honestly don't like), but I'm talking about the destination CID on
> > > which to do connect().
> > >
> > > > > so that the guest knows which device to use depending on the destination
> > > > > CID.
> > > >
> > > > Yes, this is what I was describing in the previous comment. The message
> > > > will be directed to the device by the destination CID.
> > >
> > > Sorry, I don't understand how you do this without having an
> > > information from the device about what addresses it supports. Can you
> > > elaborate a bit?
> >
> > Thanks for your explanation. So things you were talking about are as
> > follows:
> >
> > 1) guest app as a server: the app MUST do `bind()` to a CID that is
> > available in current VM.
> > 2) guest app as a client: the guest driver picks a device and uses the
> > device's CID as src CID, so that the guest app don't need to do `bind()`,
> > but only do `connect()`.
> >
> > The key point is who takes responsibility for picking a device:
> >
> > 1) I prefer the guest app to do such thing: do `bind()` to pick one, then
> > do `connect()`.
>
> Why?
>
> This implies that users must be informed in some way that they must
> use a certain CID to communicate with the VMM and another to
> communicate with the host application, when they could just as well
> use CID_HYPERVISOS(0) or CID_HOST(2) for that and everything would be
> transparent.
Sorry, I didn't express it clearly. I just listed my original idea and your
idea.
What types of services can be considered as hypervisor services? In our
case, the kata-agent is a control service for Kata, so could we consider it
as a hypervisor service?
How does the driver know which device is for hypervisor? A possible way is
to add a feature, like `VIRTIO_VSOCK_F_HYPERVISOR`? If it is possible, I
think the CID_HYPERVISOR can still be used in the case of 3+ devices to
maintain the same behavior.
> > 2) You prefer the guest driver: only do `connect()`, and the guest driver
> > picks one according to the dst CID.
> >
> > Am I right?
>
> Yes, I prefer to keep things simple.
>
> Obviously what I said works if we only have 2 devices (or at most 3
> where one supports only CID_HYPERVISOR, the other only CID_HOST, and
> the third CID >=3).
> As I anticipated, if we want to support more, then we have to
> necessarily go with the original option of this proposal with the
> default device, etc.
>
> That's why IMO we should support both.
>
> >
> > I'm open to both ideas, but I have some concerns:
> >
> > 1) Two devices are in the different namespaces, e.g. host kernel(vhost) and
> > hybrid vsock(vhost-user), which might cause two same CIDs (e.g.
> > VMADDR_CID_HOST). If that happened, the driver can't distinguish them.
> > Instead, we can avoid this by letting the guest app pick a device.
>
> Yep, in this case you need that, but how the user knows which source CID
> to use?
The CID should be confirmed in advance, and users should ensure that there
are no conflicts with CIDs.
> Again I'm not saying we should not implement this proposal, I'm saying
> that we should also add the other option in order to keep vsock simple
> when you have 2 devices, one used to talk with the VMM and the other
> to talk with applications.
Got it. Thanks for clarifying this.
> > 2) What if the number of VMs is too large? For instance, 1,000 VMs (1,000
> > CIDs) will need at least 8000B of config space. (Hmm, it looks like an
> > extreme example, I don't know if it will happen in real world.)
>
> I didn't get sorry, can you elaborate a bit?
I misunderstood. I thought you are trying to have an array to save all
supported dst CIDs in the case of 3+ devices. Please just ignore this.
> Anyway, for me we can also go ahead with this proposal and add the
> other one later.
> I would like to keep the guest simple, and IMO hiding the complexity
> in the driver is not bad for your use case where one device talks to
> the VMM and the other talks to applications in the host.
Fair enough. I am very glad to see that we can move forward with this
feature ;)
Big thanks to you!
Xuewei
> Thanks,
> Stefano
next prev parent reply other threads:[~2025-04-10 3:06 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-03-24 6:43 [PATCH v6 RESEND] virtio-vsock: Add support for multi devices Xuewei Niu
2025-03-24 13:51 ` Stefano Garzarella
2025-03-25 3:19 ` Xuewei Niu
2025-03-26 8:50 ` Stefano Garzarella
2025-03-26 10:00 ` Xuewei Niu
2025-03-26 10:32 ` Stefano Garzarella
2025-03-26 10:36 ` Stefano Garzarella
2025-03-26 2:59 ` Xuewei Niu
2025-03-26 9:03 ` Stefano Garzarella
2025-03-27 8:18 ` Xuewei Niu
2025-03-31 6:18 ` Xuewei Niu
2025-04-01 11:15 ` Stefano Garzarella
2025-04-07 2:17 ` Xuewei Niu
2025-04-08 13:34 ` Stefano Garzarella
2025-04-09 6:55 ` Xuewei Niu
2025-04-09 9:34 ` Stefano Garzarella
2025-04-10 3:05 ` Xuewei Niu [this message]
2025-04-10 7:21 ` Stefano Garzarella
2025-04-10 8:58 ` Xuewei Niu
2025-04-10 10:38 ` Stefano Garzarella
2025-04-10 10:47 ` Xuewei Niu
2025-04-10 10:49 ` Stefano Garzarella
2025-04-10 13:47 ` Michael S. Tsirkin
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