Re: [Qemu-devel] [RFC PATCH V2 0/3] IXGBE/VFIO: Add live migration support for SRIOV NIC

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Dec 01, 2015 at 09:04:32AM -0800, Alexander Duyck wrote:
> On Tue, Dec 1, 2015 at 7:28 AM, Michael S. Tsirkin <mst@redhat.com> wrote:
> > On Tue, Dec 01, 2015 at 11:04:31PM +0800, Lan, Tianyu wrote:
> >>
> >>
> >> On 12/1/2015 12:07 AM, Alexander Duyck wrote:
> >> >They can only be corrected if the underlying assumptions are correct
> >> >and they aren't.  Your solution would have never worked correctly.
> >> >The problem is you assume you can keep the device running when you are
> >> >migrating and you simply cannot.  At some point you will always have
> >> >to stop the device in order to complete the migration, and you cannot
> >> >stop it before you have stopped your page tracking mechanism.  So
> >> >unless the platform has an IOMMU that is somehow taking part in the
> >> >dirty page tracking you will not be able to stop the guest and then
> >> >the device, it will have to be the device and then the guest.
> >> >
> >> >>>Doing suspend and resume() may help to do migration easily but some
> >> >>>devices requires low service down time. Especially network and I got
> >> >>>that some cloud company promised less than 500ms network service downtime.
> >> >Honestly focusing on the downtime is getting the cart ahead of the
> >> >horse.  First you need to be able to do this without corrupting system
> >> >memory and regardless of the state of the device.  You haven't even
> >> >gotten to that state yet.  Last I knew the device had to be up in
> >> >order for your migration to even work.
> >>
> >> I think the issue is that the content of rx package delivered to stack maybe
> >> changed during migration because the piece of memory won't be migrated to
> >> new machine. This may confuse applications or stack. Current dummy write
> >> solution can ensure the content of package won't change after doing dummy
> >> write while the content maybe not received data if migration happens before
> >> that point. We can recheck the content via checksum or crc in the protocol
> >> after dummy write to ensure the content is what VF received. I think stack
> >> has already done such checks and the package will be abandoned if failed to
> >> pass through the check.
> >
> >
> > Most people nowdays rely on hardware checksums so I don't think this can
> > fly.
> 
> Correct.  The checksum/crc approach will not work since it is possible
> for a checksum to even be mangled in the case of some features such as
> LRO or GRO.
> 
> >> Another way is to tell all memory driver are using to Qemu and let Qemu to
> >> migrate these memory after stopping VCPU and the device. This seems safe but
> >> implementation maybe complex.
> >
> > Not really 100% safe.  See below.
> >
> > I think hiding these details behind dma_* API does have
> > some appeal. In any case, it gives us a good
> > terminology as it covers what most drivers do.
> 
> That was kind of my thought.  If we were to build our own
> dma_mark_clean() type function that will mark the DMA region dirty on
> sync or unmap then that is half the battle right there as we would be
> able to at least keep the regions consistent after they have left the
> driver.
> 
> > There are several components to this:
> > - dma_map_* needs to prevent page from
> >   being migrated while device is running.
> >   For example, expose some kind of bitmap from guest
> >   to host, set bit there while page is mapped.
> >   What happens if we stop the guest and some
> >   bits are still set? See dma_alloc_coherent below
> >   for some ideas.
> 
> Yeah, I could see something like this working.  Maybe we could do
> something like what was done for the NX bit and make use of the upper
> order bits beyond the limits of the memory range to mark pages as
> non-migratable?
> 
> I'm curious.  What we have with a DMA mapped region is essentially
> shared memory between the guest and the device.  How would we resolve
> something like this with IVSHMEM, or are we blocked there as well in
> terms of migration?

I have some ideas. Will post later.

> > - dma_unmap_* needs to mark page as dirty
> >   This can be done by writing into a page.
> >
> > - dma_sync_* needs to mark page as dirty
> >   This is trickier as we can not change the data.
> >   One solution is using atomics.
> >   For example:
> >         int x = ACCESS_ONCE(*p);
> >         cmpxchg(p, x, x);
> >   Seems to do a write without changing page
> >   contents.
> 
> Like I said we can probably kill 2 birds with one stone by just
> implementing our own dma_mark_clean() for x86 virtualized
> environments.
> 
> I'd say we could take your solution one step further and just use 0
> instead of bothering to read the value.  After all it won't write the
> area if the value at the offset is not 0.

Really almost any atomic that has no side effect will do.
atomic or with 0
atomic and with ffffffff

It's just that cmpxchg already happens to have a portable
wrapper.

> The only downside is that
> this is a locked operation so we will take a pretty serious
> performance penalty when this is active.  As such my preference would
> be to hide the code behind some static key that we could then switch
> on in the event of a VM being migrated.



> > - dma_alloc_coherent memory (e.g. device rings)
> >   must be migrated after device stopped modifying it.
> >   Just stopping the VCPU is not enough:
> >   you must make sure device is not changing it.
> >
> >   Or maybe the device has some kind of ring flush operation,
> >   if there was a reasonably portable way to do this
> >   (e.g. a flush capability could maybe be added to SRIOV)
> >   then hypervisor could do this.
> 
> This is where things start to get messy. I was suggesting the
> suspend/resume to resolve this bit, but it might be possible to also
> deal with this via something like this via clearing the bus master
> enable bit for the VF.  If I am not mistaken that should disable MSI-X
> interrupts and halt any DMA.  That should work as long as you have
> some mechanism that is tracking the pages in use for DMA.

A bigger issue is recovering afterwards.

> >   With existing devices,
> >   either do it after device reset, or disable
> >   memory access in the IOMMU. Maybe both.
> 
> The problem is that disabling the device at the IOMMU will start to
> trigger master abort errors when it tries to access regions it no
> longer has access to.
> 
> >   In case you need to resume on source, you
> >   really need to follow the same path
> >   as on destination, preferably detecting
> >   device reset and restoring the device
> >   state.
> 
> The problem with detecting the reset is that you would likely have to
> be polling to do something like that.

We could some event to guest to notify it about this event
through a new or existing channel.

Or we could make it possible for userspace to trigger this,
then notify guest through the guest agent.

>  I believe the fm10k driver
> already has code like that in place where it will detect a reset as a
> part of its watchdog, however the response time is something like 2
> seconds for that.  That was one of the reasons I preferred something
> like hot-plug as that should be functioning as soon as the guest is up
> and it is a mechanism that operates outside of the VF drivers.

That's pretty minor.
A bigger issue is making sure guest does not crash
when device is suddenly reset under it's legs.

-- 
MST