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    SubjectRe: [RFC PATCH 0/3] x86: Add support for guest DMA dirty page tracking
    * Michael S. Tsirkin (mst@redhat.com) wrote:
    > 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".


    Doing a hot-unplug is going to upset the guests network stacks view
    of the world; that's something we don't want to change.

    Dave

    >
    > --
    > MST
    --
    Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK


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