Re: [Xen-devel] [RFC] PVFB: Add refresh period to XenStore parameters?

[Markus Armbruster <armbru@xxxxxxxxxx>]

Samuel Thibault <samuel.thibault@xxxxxxxxxxxxx> writes:

> Markus Armbruster, le Wed 07 May 2008 16:43:38 +0200, a écrit :
>> Why do you need REQUEST_UPDATE?  Perhaps because your frontend doesn't
>> want to keep the shared framebuffer up-to-date?
>
> Yes, because it is expensive.
>
> Samuel

Strictly speaking, frame buffer update and update notification events
are separate things.

The PVFB protocol (tacitly) assumes that the framebuffer shared by the
frontend gets updated as the guest draws in it.  The update
notification event is (designed to be) optional.  The backend doesn't
actually implement the optionality, it simply bails out when it can't
get update notifications.

What you seem to need is *not* a way to control *notifications*, but a
way to control *updates*.  Because your shared framebuffer isn't
really a framebuffer, but some shadow of the real framebuffer.
Correct?

This is of course all semantic fine print, but getting that wrong can
be very confusing.

So, if I'm guessing right and you need to control updates, then what
about this: have an fb in event to advise on updates.  It contains a
suggested update frequency.  Frontends that always keep the
framebuffer in sync ignore it.  Frontends that don't keep it in sync
should immediately update it (and send an update notification for
that), and use the update frequency to guide updates until further
notice.

One could put the area to be updated into the event, but I can't see
practical applications for that.

I believe this is pretty close to what you have in mind.  But I could
be wrong.

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