From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
To: Gerd Hoffmann <kraxel@redhat.com>
Cc: Frank Yang <lfy@google.com>,
"Michael S. Tsirkin" <mst@redhat.com>,
Roman Kiryanov <rkir@google.com>,
Stefan Hajnoczi <stefanha@redhat.com>,
virtio-dev@lists.oasis-open.org
Subject: Re: [virtio-dev] Memory sharing device
Date: Thu, 21 Feb 2019 09:24:55 +0000 [thread overview]
Message-ID: <20190221092455.GB2605@work-vm> (raw)
In-Reply-To: <20190221072950.tcii5np5dsk4wvyd@sirius.home.kraxel.org>
* Gerd Hoffmann (kraxel@redhat.com) wrote:
> Hi,
>
> > > For communication between guest processes within the same VM I don't
> > > really see a need to involve the hypervisor ...
> > >
> > Right, once the host memory is set up we can rely on purely guest side stuff
> > map sub-regions of it.
>
> Or just use guest ram ...
>
> > > > Yes, also, other devices of the same VM.
> > >
> > > So why involve the hypervisor here? The guest can handle that on its
> > > own. Passing an image data buffer from the usb webcam to the intel gpu
> > > for display (on bare metal) isn't fundamentally different from passing a
> > > buffer from virtio-camera to virtio-gpu (in a VM). Linux guests will
> > > use dma-bufs for that, other OSes probably something else.
> >
> > That's true that it can be handled purely in the guest layers,
> > if there is an existing interface in the guest
> > to pass the proposed host memory id's / offsets / sizes
> > between them.
>
> Note: I think using a pci memory bar (aka host memory mapped into the
> guest) as backing storage for dma-bufs isn't going to work.
(Not knowing dma-bufs) but could you explain why?
Note in my spec the pci-bar isn't necessarily one chunk of host memory;
it's a chunk of host VMA into which multiple mmaps go.
Dave
> > However, for the proposed host memory sharing spec,
> > would there be a standard way to share the host memory across
> > different virtio devices without relying on Linux dmabufs?
>
> I think with the current draft for each device (virtio-fs, virtio-gpu,
> ...) has its own device-specific memory, and there is no mechanism to
> exchange buffers between devices.
>
> Stefan?
>
> I'm also not convinced that explicitly avoiding dmabufs is a good idea
> here. That would put virtio into its own universe and sharing buffers
> with non-virtio devices will not work. Think about a intel vgpu as
> display device, or a usb camera attached to the guest using usb
> pass-through.
>
> Experience shows that using virtualization-specific features /
> optimizations / short-cuts often turns out to have drawbacks in the long
> run, even if it looked like a good idea initially. Just look at the
> mess we had with virtio-pci dma after iommu emulation landed in qemu.
> And this is only one example, we have more of this ...
>
> cheers,
> Gerd
>
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
---------------------------------------------------------------------
To unsubscribe, e-mail: virtio-dev-unsubscribe@lists.oasis-open.org
For additional commands, e-mail: virtio-dev-help@lists.oasis-open.org
next prev parent reply other threads:[~2019-02-21 9:25 UTC|newest]
Thread overview: 72+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-02-01 20:34 [virtio-dev] Memory sharing device Roman Kiryanov
2019-02-04 5:40 ` Stefan Hajnoczi
2019-02-04 10:13 ` Gerd Hoffmann
2019-02-04 10:18 ` Roman Kiryanov
2019-02-05 7:42 ` Roman Kiryanov
2019-02-05 10:04 ` Dr. David Alan Gilbert
2019-02-05 15:17 ` Frank Yang
2019-02-05 15:21 ` Frank Yang
2019-02-05 21:06 ` Roman Kiryanov
2019-02-06 7:03 ` Gerd Hoffmann
2019-02-06 15:09 ` Frank Yang
2019-02-06 15:11 ` Frank Yang
2019-02-08 7:57 ` Stefan Hajnoczi
2019-02-08 14:46 ` Frank Yang
2019-02-06 20:14 ` Dr. David Alan Gilbert
2019-02-06 20:27 ` Frank Yang
2019-02-07 12:10 ` Cornelia Huck
2019-02-11 14:49 ` Michael S. Tsirkin
2019-02-11 15:14 ` Frank Yang
2019-02-11 15:25 ` Frank Yang
2019-02-12 13:01 ` Michael S. Tsirkin
2019-02-12 13:16 ` Dr. David Alan Gilbert
2019-02-12 13:27 ` Michael S. Tsirkin
2019-02-12 16:17 ` Frank Yang
2019-02-19 7:17 ` Gerd Hoffmann
2019-02-19 15:59 ` Frank Yang
2019-02-20 6:51 ` Gerd Hoffmann
2019-02-20 15:31 ` Frank Yang
2019-02-21 6:55 ` Gerd Hoffmann
2019-02-19 7:12 ` Gerd Hoffmann
2019-02-19 16:02 ` Frank Yang
2019-02-20 7:02 ` Gerd Hoffmann
2019-02-20 15:32 ` Frank Yang
2019-02-21 7:29 ` Gerd Hoffmann
2019-02-21 9:24 ` Dr. David Alan Gilbert [this message]
2019-02-21 9:59 ` Gerd Hoffmann
2019-02-21 10:03 ` Dr. David Alan Gilbert
2019-02-22 6:15 ` Michael S. Tsirkin
2019-02-22 6:42 ` Gerd Hoffmann
2019-02-11 16:57 ` Michael S. Tsirkin
2019-02-12 8:27 ` Roman Kiryanov
2019-02-12 11:25 ` Dr. David Alan Gilbert
2019-02-12 13:47 ` Cornelia Huck
2019-02-12 14:03 ` Michael S. Tsirkin
2019-02-12 15:56 ` Frank Yang
2019-02-12 16:46 ` Dr. David Alan Gilbert
2019-02-12 17:20 ` Frank Yang
2019-02-12 17:26 ` Frank Yang
2019-02-12 19:06 ` Michael S. Tsirkin
2019-02-13 2:50 ` Frank Yang
2019-02-13 4:02 ` Michael S. Tsirkin
2019-02-13 4:19 ` Michael S. Tsirkin
2019-02-13 4:59 ` Frank Yang
2019-02-13 18:18 ` Frank Yang
2019-02-14 7:15 ` Frank Yang
2019-02-22 22:05 ` Michael S. Tsirkin
2019-02-24 21:19 ` Frank Yang
2019-02-13 4:59 ` Frank Yang
2019-02-19 7:54 ` Gerd Hoffmann
2019-02-19 15:54 ` Frank Yang
2019-02-20 3:46 ` Michael S. Tsirkin
2019-02-20 15:24 ` Frank Yang
2019-02-20 19:29 ` Michael S. Tsirkin
2019-02-20 6:25 ` Gerd Hoffmann
2019-02-20 15:30 ` Frank Yang
2019-02-20 15:35 ` Frank Yang
2019-02-21 6:44 ` Gerd Hoffmann
2019-02-12 18:22 ` Michael S. Tsirkin
2019-02-12 19:01 ` Frank Yang
2019-02-12 19:15 ` Michael S. Tsirkin
2019-02-12 20:15 ` Frank Yang
2019-02-12 13:00 ` Michael S. Tsirkin
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20190221092455.GB2605@work-vm \
--to=dgilbert@redhat.com \
--cc=kraxel@redhat.com \
--cc=lfy@google.com \
--cc=mst@redhat.com \
--cc=rkir@google.com \
--cc=stefanha@redhat.com \
--cc=virtio-dev@lists.oasis-open.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox