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virtio-balloon: fix QEMU 4.0 config size migration incompatibility
The virtio-balloon config size changed in QEMU 4.0 even for existing machine types. Migration from QEMU 3.1 to 4.0 can fail in some circumstances with the following error: qemu-system-x86_64: get_pci_config_device: Bad config data: i=0x10 read: a1 device: 1 cmask: ff wmask: c0 w1cmask:0 This happens because the virtio-balloon config size affects the VIRTIO Legacy I/O Memory PCI BAR size. Introduce a qdev property called "qemu-4-0-config-size" and enable it only for the QEMU 4.0 machine types. This way <4.0 machine types use the old size, 4.0 uses the larger size, and >4.0 machine types use the appropriate size depending on enabled virtio-balloon features. Live migration to and from old QEMUs to QEMU 4.1 works again as long as a versioned machine type is specified (do not use just "pc"!). Originally-by: Wolfgang Bumiller <w.bumiller@proxmox.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com> Message-Id: <20190710141440.27635-1-stefanha@redhat.com> Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com> Tested-by: Dr. David Alan Gilbert <dgilbert@redhat.com> Tested-by: Wolfgang Bumiller <w.bumiller@proxmox.com> Reviewed-by: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
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