Re: [Qemu-devel] [PATCH 1/2] vhost-user-blk: introduce a new vhost-user-blk host device

[Stefan Hajnoczi]

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.

It's worth checking how virtio config space is live migrated and how to
do that consistently if the vhost-user process is involved.

Stefan

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