Re: [Qemu-devel] [PATCH 0/5] QEMU VFIO live migration

[Gonglei (Arei)]

> -----Original Message-----
> From: Zhao Yan [mailto:address@hidden
> Sent: Thursday, February 21, 2019 9:59 AM
> To: Gonglei (Arei) <address@hidden>
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> Subject: Re: [PATCH 0/5] QEMU VFIO live migration
> 
> On Thu, Feb 21, 2019 at 01:35:43AM +0000, Gonglei (Arei) wrote:
> >
> >
> > > -----Original Message-----
> > > From: Zhao Yan [mailto:address@hidden
> > > Sent: Thursday, February 21, 2019 8:25 AM
> > > To: Gonglei (Arei) <address@hidden>
> > > Cc: address@hidden; address@hidden;
> > > address@hidden; address@hidden;
> > > address@hidden; address@hidden; address@hidden;
> > > address@hidden; address@hidden;
> address@hidden;
> > > address@hidden; address@hidden; address@hidden;
> > > address@hidden; address@hidden; address@hidden;
> > > address@hidden; address@hidden;
> address@hidden;
> > > address@hidden; address@hidden; address@hidden;
> > > address@hidden
> > > Subject: Re: [PATCH 0/5] QEMU VFIO live migration
> > >
> > > On Wed, Feb 20, 2019 at 11:56:01AM +0000, Gonglei (Arei) wrote:
> > > > Hi yan,
> > > >
> > > > Thanks for your work.
> > > >
> > > > I have some suggestions or questions:
> > > >
> > > > 1) Would you add msix mode support,? if not, pls add a check in
> > > vfio_pci_save_config(), likes Nvidia's solution.
> > > ok.
> > >
> > > > 2) We should start vfio devices before vcpu resumes, so we can't rely on
> vm
> > > start change handler completely.
> > > vfio devices is by default set to running state.
> > > In the target machine, its state transition flow is 
> > > running->stop->running.
> >
> > That's confusing. We should start vfio devices after vfio_load_state,
> otherwise
> > how can you keep the devices' information are the same between source side
> > and destination side?
> >
> so, your meaning is to set device state to running in the first call to
> vfio_load_state?
> 
No, it should start devices after vfio_load_state and before vcpu resuming.

> > > so, maybe you can ignore the stop notification in kernel?
> > > > 3) We'd better support live migration rollback since have many failure
> > > scenarios,
> > > >  register a migration notifier is a good choice.
> > > I think this patchset can also handle the failure case well.
> > > if migration failure or cancelling happens,
> > > in cleanup handler, LOGGING state is cleared. device state(running or
> > > stopped) keeps as it is).
> >
> > IIRC there're many failure paths don't calling cleanup handler.
> >
> could you take an example?

Never mind, that's another bug I think. 

> > > then,
> > > if vm switches back to running, device state will be set to running;
> > > if vm stayes at stopped state, device state is also stopped (it has no
> > > meaning to let it in running state).
> > > Do you think so ?
> > >
> > IF the underlying state machine is complicated,
> > We should tell the canceling state to vendor driver proactively.
> >
> That makes sense.
> 
> > > > 4) Four memory region for live migration is too complicated IMHO.
> > > one big region requires the sub-regions well padded.
> > > like for the first control fields, they have to be padded to 4K.
> > > the same for other data fields.
> > > Otherwise, mmap simply fails, because the start-offset and size for mmap
> > > both need to be PAGE aligned.
> > >
> > But if we don't need use mmap for control filed and device state, they are
> small basically.
> > The performance is enough using pread/pwrite.
> >
> we don't mmap control fields. but if data fields going immedately after
> control fields (e.g. just 64 bytes), we can't mmap data fields
> successfully because its start offset is 64. Therefore control fields have
> to be padded to 4k to let data fields start from 4k.
> That's the drawback of one big region holding both control and data fields.
> 
> > > Also, 4 regions is clearer in my view :)
> > >
> > > > 5) About log sync, why not register log_global_start/stop in
> > > vfio_memory_listener?
> > > >
> > > >
> > > seems log_global_start/stop cannot be iterately called in pre-copy phase?
> > > for dirty pages in system memory, it's better to transfer dirty data
> > > iteratively to reduce down time, right?
> > >
> >
> > We just need invoking only once for start and stop logging. Why we need to
> call
> > them literately? See memory_listener of vhost.
> >
> 
> 
> 
> > Regards,
> > -Gonglei