Re: [Qemu-devel] [PATCH 1/2] char: add qemu_chr_be_is_fe_connected

[Hans de Goede <hdegoede@redhat.com>]

Hi,

On 03/22/2013 09:25 AM, Gerd Hoffmann wrote:
>    Hi,
>
>> There isn't a notion of "connected" for the front-ends in the char
>> layer.  The closest thing is whether add_handlers() have been called or
>> not.
>
> It isn't new.  There are qemu_chr_fe_open + qemu_chr_fe_close doing
> exactly that signaling (whenever someone has opened the virtio-serial
> port, i.e. whenever the guest agent is active or not).
>
> Problem is the chardev forgets this state over migration.
>
> So we need a way to restore the state.  virtio-serial knows it of course
> as this is guest state it has to keep track of it.  We just need a way
> the chardev can figure what the current state is after migration.
>
> The options I see:
>
>    (1) Have chardev store the state itself in a new migration section.
>        This is what I've NAK'ed.  First, because live-migration host
>        state is a can of worms I don't want open.  Second because it
>        actually isn't host state.
>
>    (2) virtio-serial could just call qemu_chr_fe_open in
>        post_migration hook.  Could have unwanted side effects as
>        the chardev can't figure whenever this is a post-load call
>        or a guest-open call.
>
>    (3) Add a new qemu_chr_fe_* call for virtio-serial to notify the
>        chardev.  This was the next proposal from Alon
>
>    (4) Do it the other way around:  Allow the chardev query what the
>        current state is.  This patch series.
>

Thanks for the above overview, very useful!

>    (5) Cut off the chardev layer altogether and implement spicevmc as
>        virtio-serial port.  Your proposal if I understand it correctly.
>        Makes sense, but breaks backward compatibility, so even when
>        doing this we must fix the chardev spicevmc bug somehow.

I've to nack this one, spicevmc is a chardev backend which is also used
with other frontends then just virtio-console, atm it is also used together
with the usb-redir and smartcard frontends and there is no reason why it
could not be used with others in the future, so: NACK.

Regards,

Hans