Hi Peter
On 2025-05-21 12:04, Peter Xu wrote:
> On Wed, May 21, 2025 at 05:16:13PM +0200, Juraj Marcin wrote:
> > From: Juraj Marcin <jmar...@redhat.com>
> >
> > If a virtual machine is paused for an extended period time, for example,
> > due to an incoming migration, there are also no changes on the screen.
> > VNC in such case increases the display update interval by
> > VNC_REFRESH_INTERVAL_INC (50 ms). The update interval can then grow up
> > to VNC_REFRESH_INTERVAL_MAX (3000 ms).
> >
> > When the machine resumes, it can then take up to 3 seconds for the first
> > display update. Furthermore, the update interval is then halved with
> > each display update with changes on the screen. If there are moving
> > elements on the screen, such as a video, this can be perceived as
> > freezing and stuttering for few seconds before the movement is smooth
> > again.
> >
> > This patch resolves this issue, by adding a listener to VM state changes
> > and changing the update interval when the VM state changes to RUNNING.
> > The update_displaychangelistener() function updates the internal timer,
> > and the display is refreshed immediately if the timer is expired.
> >
> > Signed-off-by: Juraj Marcin <jmar...@redhat.com>
>
> Thanks for looking into it!
>
> Reviewed-by: Peter Xu <pet...@redhat.com>
>
> One trivial comment (and partly, pure question) below,
>
> > ---
> > ui/vnc.c | 12 ++++++++++++
> > ui/vnc.h | 2 ++
> > 2 files changed, 14 insertions(+)
> >
> > diff --git a/ui/vnc.c b/ui/vnc.c
> > index 9e097dc4b4..32f8bfd1f9 100644
> > --- a/ui/vnc.c
> > +++ b/ui/vnc.c
> > @@ -3384,6 +3384,16 @@ static const DisplayChangeListenerOps dcl_ops = {
> > .dpy_cursor_define = vnc_dpy_cursor_define,
> > };
> >
> > +static void vmstate_change_handler(void *opaque, bool running, RunState
> > state)
> > +{
> > + VncDisplay *vd = opaque;
> > +
> > + if (state != RUN_STATE_RUNNING) {
>
> Just to mention in vm_prepare_start() it's possible we migrate a VM that
> used to be suspended, if so it'll keep suspended after migration:
>
> RunState state = vm_was_suspended ? RUN_STATE_SUSPENDED :
> RUN_STATE_RUNNING;
>
> Here I'm not sure whether SUSPENDED would also like to update the display
> freq. I don't think it matters hugely, but just to say, if we want we can
> simply check "running=true" instead of checking the state to cover both
> RUNNING|SUSPENDED cases.
Thank you for the comment. I don't think it is necessary to update the
screen frequency if the machine is suspended. In case there is an
explicit request for that, we can change it then. The display frequency
is still updated when the machine is resumed, same as if it was just
suspended and then resumed without migration.
Best regards
Juraj Marcin
>
> > + return;
> > + }
> > + update_displaychangelistener(&vd->dcl, VNC_REFRESH_INTERVAL_BASE);
> > +}
> > +
> > void vnc_display_init(const char *id, Error **errp)
> > {
> > VncDisplay *vd;
> > @@ -3420,6 +3430,8 @@ void vnc_display_init(const char *id, Error **errp)
> > vd->dcl.ops = &dcl_ops;
> > register_displaychangelistener(&vd->dcl);
> > vd->kbd = qkbd_state_init(vd->dcl.con);
> > + vd->vmstate_handler_entry = qemu_add_vm_change_state_handler(
> > + &vmstate_change_handler, vd);
> > }
> >
> >
> > diff --git a/ui/vnc.h b/ui/vnc.h
> > index acc53a2cc1..3bb23acd34 100644
> > --- a/ui/vnc.h
> > +++ b/ui/vnc.h
> > @@ -185,6 +185,8 @@ struct VncDisplay
> > #endif
> >
> > AudioState *audio_state;
> > +
> > + VMChangeStateEntry *vmstate_handler_entry;
> > };
> >
> > typedef struct VncTight {
> > --
> > 2.49.0
> >
>
> --
> Peter Xu
>
