Re: [Qemu-devel] [PATCH 2/2] atapi migration: Throw recoverable error to avoid recovery

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Dr. David Alan Gilbert (dgilbert@redhat.com) wrote:
> * Paolo Bonzini (pbonzini@redhat.com) wrote:
> > 
> > 
> > On 09/12/2014 19:15, Dr. David Alan Gilbert (git) wrote:
> > > (With the previous atapi_dma flag recovery)
> > > If migration happens between the ATAPI command being written and the
> > > bmdma being started, the DMA is dropped.  Eventually the guest times
> > > out and recovers, but that can take many seconds.
> > > (This is rare, on a pingpong reading the CD continuously I hit
> > > this about ~1/30-1/50 migrates)
> > > 
> > > I don't think we've got enough state to be able to recover safely
> > > at this point, so I throw a 'medium error, no seek complete'
> > > that I'm assuming guests will try and recover from an apparently
> > > dirty CD.
> > 
> 
> > Also, what is the ATAPI command that is being run?
> 
> The command in my test case is:
> 28 00 00 06 df b8 00 00 40 00 00 00
> so just a standard READ (10); I guess it's possible that other OSs/operations
> maybe doing different READ variants.
> 
> > What is the part of the state that is not being migrated?
> 
> I can't see lba, io_buffer_index, io_buffer_size or cd_sector_size being passed
> to a VMSTATE_ macro; and having noticed that those are missing I didn't dig much
> deeper.
> (I'll admit to not really understanding the interaction between the core.c, atapi.c
> and pci.c dma code properly to know if it can be reconstructed).

Actually, another argument for this is that even if we come along and add
the extra state as a subsection, we can still do this as a fallback when receiving
older streams.

Dave
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK