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Re: [Qemu-block] [Qemu-devel] [PATCH 2/2] scsi: add block job opblockers

From: Kevin Wolf
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH 2/2] scsi: add block job opblockers for scsi-block
Date: Mon, 12 Mar 2018 12:58:46 +0100
User-agent: Mutt/1.9.1 (2017-09-22)

Am 12.03.2018 um 12:10 hat Paolo Bonzini geschrieben:
> On 12/02/2018 15:50, Paolo Bonzini wrote:
> > On 12/02/2018 15:48, Kevin Wolf wrote:
> >> Am 12.02.2018 um 15:32 hat Paolo Bonzini geschrieben:
> >>> Okay, we are in agreement about this and you expressed very well why I
> >>> (at the gut feeling level) didn't like the old op blockers.  But you
> >>> bypassed the real question, which is: should I send a pull request for
> >>> these two patches or not? :)
> >> I didn't spell it out that explicitly, but this is essentially a NACK.
> >> I'd very much prefer if you could replace it with the proper solution.
> >> Of course, we can always make exceptions when there is a good reason,
> >> but with 2.12 still two months away, I doubt we have one.
> > Ok, I don't mind explicitness.  I'll keep these two patches in the queue
> > for now.
> It's now one month away.  Regarding the solution below:
> > I propose a new BLK_PERM_BYPASS that allows its users to bypass the
> > block layer I/O functions. In other words, bdrv_aio_ioctl() would
> > require that you got this permission. A dirty bitmap would keep a
> > BdrvChild with perm=0, shared=BLK_PERM_ALL & ~BLK_PERM_BYPASS, so you
> > can never have a dirty bitmap and a device using ioctls attached to the
> > BDS at the same time.
> I suppose it would be like:
> - scsi-block/scsi-generic call blk_set_perm with perm == shared ==

perm = BLK_PERM_BYPASS is fine, but for shared it seems overly
restrictive. I don't think the device minds another user accessing the

Other block devices do this in blkconf_apply_backend_options():

    if (resizable) {
        shared_perm |= BLK_PERM_RESIZE;
    if (conf->share_rw) {
        shared_perm |= BLK_PERM_WRITE;

I suppose scsi-generic is never resizable, so that part can go away, but
we do have a share-rw qdev property that can be used.

> - users of dirty bitmaps would call use perm/shared_perm as in your
> message above
> - dirty bitmaps creation calls bdrv_get_cumulative_perm (which should
> now become public) and checks that it doesn't have BLK_PERM_BYPASS in
> shared_perm

My proposal was really that users of dirty bitmaps don't change
anything, but we do everything in the dirty bitmap implementation. Dirty
bitmap creation would add a BdrvChild with the above permissions.
Deleting a dirty bitmap would remove the BdrvChild again.

Then you don't need to manually call bdrv_get_cumulative_perm(), because
the permission check is included when you attach the BdrvChild.

> Anything I'm missing?

Ideally, bdrv_co_ioctl() should take a BdrvChild instead of a BDS and
assert that the caller correctly requested the permission:

    assert(child->perm & BLK_PERM_BYPASS);


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