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Re: [Qemu-devel] [PATCH] input: Add trace events for polled keyboard inp

From: Alexander Graf
Subject: Re: [Qemu-devel] [PATCH] input: Add trace events for polled keyboard input
Date: Thu, 30 Mar 2017 16:12:43 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.6.0

On 03/30/2017 04:00 PM, Gerd Hoffmann wrote:
On Do, 2017-03-30 at 15:35 +0200, Alexander Graf wrote:
When driving QEMU from the outside, we have basically no chance to
determine how quickly the guest OS picks up key events, so we usually
have to limit ourselves to very slow keyboard presses to make sure
the guest always has enough chance to pick them up.

This patch adds trace events for when the guest polls for HID keyboard
events. That way we can be reasonably safely assume that the guest
handled that one key event and can type the next.
Hmm, I'm wondering why you are tracing the event details?  Input layer
has tracepoints for this already, so why duplicate this here?
Especially as you have to do quite some effort to store the data for

Because we need to know when the events are off the queue, not pushed onto the queue.

For queue management (in automated testing I assume?) a simple "queue
empty" trace point should do the trick, no?  Or maybe a "queue has $i of
$n slots filled atm" trace points, if you wanna know how much you can
pass in without loosing events due to the queue being full.

That's a very good point. In fact, that would make things much much easier for the other side. If we just know "queue is drained", we know we can send new events.


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