Re: [Qemu-devel] [RFC v3 0/3] scsi: restart dma after vm change state handlers

[Kevin Wolf]

Am 24.05.2019 um 20:47 hat Paolo Bonzini geschrieben:
> On 24/05/19 20:36, Stefan Hajnoczi wrote:
> > v3:
> >  * Fix s/k->vmstate_change/vdc->vmstate_change/
> >  * Still RFC, waiting for customer to confirm this fixes the issue
> > v2:
> >  * Do it properly with a clean API instead of deferring to a BH!
> >    Thanks for encouraging me to do this, Kevin.
> > 
> > These patches solve a deadlock when the 'cont' command is used and there are
> > failed requests on a virtio-scsi device with iothreads.  The deadlock 
> > itself is
> > actually not the thing we need to fix because we should never reach that 
> > case
> > anyway.  Instead we need to make sure DMA restart is only performed after 
> > the
> > virtio-scsi iothread is re-initialized.
> 
> custom_dma_restart is a bit ugly...  Do you think it would make sense to
> order the VMStateChange handlers using some kind of enum (with the order
> unspecified within the category)?
> 
> We could start with
> 
>       VMSTATECHANGE_PRIO_UNKNOWN  = 0  (if needed?)
>       VMSTATECHANGE_PRIO_IOTHREAD = 100
>         VMSTATECHANGE_PRIO_DEVICE   = 200
> 
> where higher priorities run first on stop and last on resume.

I don't think this is as nice because stopping or resuming a device
could involve some async operation (e.g. be delegated to a BH). In this
case, a device on a child bus must still wait for the BH (or other async
part) to be completed before it can resume its own operation.

Basically, a single flat list of global VM state handlers wasn't a good
design, because what we actually need in such cases is something
hierarchical.

Kevin