Re: [Qemu-block] [RFC PATCH for-2.9] block: Ignore guest dev permissions during incoming migration

[Kevin Wolf]

Am 05.04.2017 um 15:22 hat Max Reitz geschrieben:
> On 04.04.2017 17:35, Kevin Wolf wrote:
> > Usually guest devices don't like other writers to the same image, so
> > they use blk_set_perm() to prevent this from happening. In the migration
> > phase before the VM is actually running, though, they don't have a
> > problem with writes to the image. On the other hand, storage migration
> > needs to be able to write to the image in this phase, so the restrictive
> > blk_set_perm() call of qdev devices breaks it.
> > 
> > This patch flags all BlockBackends with a qdev device as
> > blk->disable_perm during incoming migration, which means that the
> > requested permissions are stored in the BlockBackend, but not actually
> > applied to its root node yet.
> > 
> > Once migration has finished and the VM should be resumed, the
> > permissions are applied. If they cannot be applied (e.g. because the NBD
> > server used for block migration hasn't been shut down), resuming the VM
> > fails.
> > 
> > Signed-off-by: Kevin Wolf <address@hidden>
> > ---
> >  block/block-backend.c | 40 +++++++++++++++++++++++++++++++++++++++-
> >  include/block/block.h |  2 ++
> >  migration/migration.c |  8 ++++++++
> >  qmp.c                 |  6 ++++++
> >  4 files changed, 55 insertions(+), 1 deletion(-)
> > 
> > diff --git a/block/block-backend.c b/block/block-backend.c
> > index 0b63773..f817040 100644
> > --- a/block/block-backend.c
> > +++ b/block/block-backend.c
> 
> [...]
> 
> > @@ -597,15 +598,52 @@ void blk_get_perm(BlockBackend *blk, uint64_t *perm, 
> > uint64_t *shared_perm)
> >      *shared_perm = blk->shared_perm;
> >  }
> >  
> > +/*
> > + * Notifies the user of all BlockBackends that migration has completed. 
> > qdev
> > + * devices can tighten their permissions in response (specifically revoke
> > + * shared write permissions that we needed for storage migration).
> > + *
> > + * If an error is returned, the VM cannot be allowed to be resumed.
> > + */
> > +void blk_resume_after_migration(Error **errp)
> > +{
> > +    BlockBackend *blk;
> > +    Error *local_err = NULL;
> > +
> > +    for (blk = blk_next(NULL); blk; blk = blk_next(blk)) {
> 
> Shouldn't we use blk_all_next()?

Good catch, thanks.

At first I added it into the loop in qmp_cont() and then copied it here
without noticing the resetting the iostatus is really only needed for
monitor-owned BBs at the moment, but this one is different.

Of course, as soon as we improve query-block to work reasonably well
with -device drive=<node-name>, qmp_cont() needs to use blk_all_next(),
too.

> Trusting you that silently disabling autostart is something the upper
> layers can deal with, the rest looks good to me.
> 
> (The only other runtime changes of autostart apart from stop/cont appear
> to be in blockdev_init() (if (bdrv_key_required()), but I don't think
> that can happen anymore) and in migration/colo.c (which enables it and
> emits an error message).)

I think in practice libvirt always sets -S on the destination anyway.

Kevin

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