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Re: [libvirt] [PATCH] Revert "qemu: propagate bridge MTU into qemu "host_mtu" option"



On Mon, May 22, 2017 at 03:30:48AM -0400, Laine Stump wrote:
> This reverts commit 2841e675.
> 
> It turns out that adding the host_mtu field to the PCI capabilities in
> the guest bumps the length of PCI capabilities beyond the 32 byte
> boundary, so the virtio-net device gets 64 bytes of ioport space
> instead of 32, which offsets the address of all the other following
> devices. Migration doesn't work very well when the location and length
> of PCI capabilities of devices is changed between source and
> destination.
> 
> This means that we need to make sure that the absence/presence of
> host_mtu on the qemu commandline always matches between source and
> destination, which means that we need to make setting of host_mtu an
> opt-in thing (it can't happen automatically when the bridge being used
> has a non-default MTU, which is what commit 2841e675 implemented).
> 
> I do want to re-implement this feature with an <mtu auto='on'/>
> setting, but probably won't backport that to any stable branches, so
> I'm first reverting the original commit, and that revert can be pushed
> to the few releases that have been made since the original (3.1.0 -
> 3.3.0)
> 
> Resolves: https://bugzilla.redhat.com/1449346
> ---
>  src/qemu/qemu_command.c   | 32 ++++++++++----------------------
>  src/qemu/qemu_command.h   |  3 +--
>  src/qemu/qemu_hotplug.c   |  5 ++---
>  src/qemu/qemu_interface.c |  5 ++---
>  src/qemu/qemu_interface.h |  3 +--
>  5 files changed, 16 insertions(+), 32 deletions(-)

ACK, please push to all relevant branches too


Regards,
Daniel
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