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Re: [Qemu-devel] [PATCH 1/2] vhost-user-blk: introduce a new vhost-user-
From: |
Stefan Hajnoczi |
Subject: |
Re: [Qemu-devel] [PATCH 1/2] vhost-user-blk: introduce a new vhost-user-blk host device |
Date: |
Fri, 28 Jul 2017 11:35:57 +0100 |
User-agent: |
Mutt/1.8.3 (2017-05-23) |
On Thu, Jul 27, 2017 at 10:08:49AM +0000, Liu, Changpeng wrote:
> > -----Original Message-----
> > From: Stefan Hajnoczi [mailto:address@hidden
> > Sent: Thursday, July 27, 2017 5:49 PM
> > To: Liu, Changpeng <address@hidden>
> > Cc: address@hidden; address@hidden; address@hidden;
> > address@hidden
> > Subject: Re: [PATCH 1/2] vhost-user-blk: introduce a new vhost-user-blk host
> > device
> >
> > On Thu, Jul 27, 2017 at 12:29:45AM +0000, Liu, Changpeng wrote:
> > > > > + .fields = (VMStateField[]) {
> > > > > + VMSTATE_VIRTIO_DEVICE,
> > > > > + VMSTATE_END_OF_LIST()
> > > > > + },
> > > > > +};
> > > > > +
> > > > > +static Property vhost_user_blk_properties[] = {
> > > > > + DEFINE_PROP_CHR("chardev", VHostUserBlk, chardev),
> > > > > + DEFINE_BLOCK_PROPERTIES(VHostUserBlk, blkcfg),
> > > >
> > > > DEFINE_BLOCK_PROPERTIES cannot be used directly because it includes the
> > > > 'drive' (blkcfg.blk) parameter. The command-line should not allow a
> > > > drive= parameter.
> > > >
> > > > Also, parameters like the discard granularity, optimal I/O size, logical
> > > > block size, etc can be initialized to sensible defaults by QEMU's block
> > > > layer when drive= is used. Since you are bypassing QEMU's block layer
> > > > there is no way for QEMU to set good defaults.
> > > >
> > > > Are you relying on the managment tools populating these parameters with
> > > > the correct values from the vhost-user-blk process? Or did you have
> > > > some other mechanism in mind?
> > > Actually for this part and your comments about block resize, I think
> > > maybe add
> > several
> > > additional vhost user messages such like:
> > VHOST_USER_GET_BLK_CFG/VHOST_USER_SET_BLK_CFG
> > > makes the code more clear, I'm okay to add such messages.
> >
> > New messages for virtio config space read/write might be useful for
> > other devices besides virtio-blk.
> I mean all the block device information like: block_size/capacity are stored
> in another process,
> so add a new vhost user message to Qemu vhost-user-blk can get those
> information when Qemu get
> started, once those messages stored to virtio_pci config space, we will not
> change it.
No, I think updates are necessary:
The virtio block device can do online disk resize, so it will be
necessary to change the capacity field from the vhost-user-blk process
at runtime and raise a config change notification interrupt.
The virtio block device also has a config space field ("wce") that is
writable by the guest. Supporting this feature also requires virtio
config space support in vhost-user.
If you focus on adding generic virtio config space messages to
vhost-user then these virtio block features can be supported by
vhost-user-blk.
Regarding the two approaches of adding "block device information" as you
have suggested versus adding generic virtio config space support as I'm
suggesting, from a protocol design perspective it's nicer to have
generic messages that are usable by all device types. I'm not aware of
a reason why high-level "block device information" is needed since QEMU
will just put that into the config space but otherwise does not
interpret the information.
> > It's worth checking how virtio config space is live migrated and how to
> > do that consistently if the vhost-user process is involved.
> Virtio will config spaces for virtio_blk, and even the config space migrated
> to another machine, it should be
> same with the another I/O process, did I get your comments ? Thanks.
The guest-writable "wce" config space field is an example that shows the
vhost-user-blk process on the destination host does not have all the
state necessary. QEMU needs to migrate the config space and send it to
the vhost-user-blk process on the destination.
Stefan
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- [Qemu-devel] [PATCH 0/2] Introduce a new vhost-user-blk device and sample application, Changpeng Liu, 2017/07/25
- [Qemu-devel] [PATCH 2/2] vhost-user-blk: introduce a vhost-user-blk sample application, Changpeng Liu, 2017/07/25
- [Qemu-devel] [PATCH 1/2] vhost-user-blk: introduce a new vhost-user-blk host device, Changpeng Liu, 2017/07/25
- Re: [Qemu-devel] [PATCH 1/2] vhost-user-blk: introduce a new vhost-user-blk host device, Stefan Hajnoczi, 2017/07/26
- Re: [Qemu-devel] [PATCH 1/2] vhost-user-blk: introduce a new vhost-user-blk host device, Liu, Changpeng, 2017/07/26
- Re: [Qemu-devel] [PATCH 1/2] vhost-user-blk: introduce a new vhost-user-blk host device, Stefan Hajnoczi, 2017/07/27
- Re: [Qemu-devel] [PATCH 1/2] vhost-user-blk: introduce a new vhost-user-blk host device, Liu, Changpeng, 2017/07/27
- Re: [Qemu-devel] [PATCH 1/2] vhost-user-blk: introduce a new vhost-user-blk host device,
Stefan Hajnoczi <=
- Re: [Qemu-devel] [PATCH 1/2] vhost-user-blk: introduce a new vhost-user-blk host device, Liu, Changpeng, 2017/07/28
- Re: [Qemu-devel] [PATCH 1/2] vhost-user-blk: introduce a new vhost-user-blk host device, Stefan Hajnoczi, 2017/07/31
- Re: [Qemu-devel] [PATCH 1/2] vhost-user-blk: introduce a new vhost-user-blk host device, Paolo Bonzini, 2017/07/31
- Re: [Qemu-devel] [PATCH 1/2] vhost-user-blk: introduce a new vhost-user-blk host device, Liu, Changpeng, 2017/07/31