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

[Zhao Yan]

On Thu, Feb 21, 2019 at 03:33:24AM +0000, Gonglei (Arei) wrote:
> 
> > -----Original Message-----
> > From: Zhao Yan [mailto:address@hidden
> > Sent: Thursday, February 21, 2019 9:59 AM
> > To: Gonglei (Arei) <address@hidden>
> > Cc: address@hidden; address@hidden;
> > address@hidden; 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.
>

What about set device state to running in load_cleanup handler ?

> > > > 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
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