Re: [RFC PATCH 0/3] x86: Add support for guest DMA dirty page tracking

[Michael S. Tsirkin]

On Mon, Jan 04, 2016 at 07:11:25PM -0800, Alexander Duyck wrote:
> >> The two mechanisms referenced above would likely require coordination with
> >> QEMU and as such are open to discussion.  I haven't attempted to address
> >> them as I am not sure there is a consensus as of yet.  My personal
> >> preference would be to add a vendor-specific configuration block to the
> >> emulated pci-bridge interfaces created by QEMU that would allow us to
> >> essentially extend shpc to support guest live migration with pass-through
> >> devices.
> >
> > shpc?
> 
> That is kind of what I was thinking.  We basically need some mechanism
> to allow for the host to ask the device to quiesce.  It has been
> proposed to possibly even look at something like an ACPI interface
> since I know ACPI is used by QEMU to manage hot-plug in the standard
> case.
> 
> - Alex


Start by using hot-unplug for this!

Really use your patch guest side, and write host side
to allow starting migration with the device, but
defer completing it.

So

1.- host tells guest to start tracking memory writes
2.- guest acks
3.- migration starts
4.- most memory is migrated
5.- host tells guest to eject device
6.- guest acks
7.- stop vm and migrate rest of state


It will already be a win since hot unplug after migration starts and
most memory has been migrated is better than hot unplug before migration
starts.

Then measure downtime and profile. Then we can look at ways
to quiesce device faster which really means step 5 is replaced
with "host tells guest to quiesce device and dirty (or just unmap!)
all memory mapped for write by device".

-- 
MST
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