Re: [Qemu-devel] User space vs kernel space instructions distribution.

[Peter Maydell]

On 22 July 2015 at 17:45, Christopher Covington <address@hidden> wrote:
> On 07/14/2015 04:45 AM, Peter Maydell wrote:
>> If your measurement instrumentation is intrusive and is significantly
>> slowing down QEMU then you'll naturally find that the guest spends
>> more time in timer interrupt handling, because the timer interrupts
>> come in in real time, and you've just effectively reduced the speed
>> of your CPU, so it can get less useful work done between timer
>> interrupts.
>
> I find such behavior undesirable. As best I understand, -icount exists to
> provide an alternative, although it may have bugs.

Well, the default is "go as fast as we can while presenting real
world time and events to the guest". That's mostly what people
want, except that it breaks down when "as fast as we can" turns
out to be "really slowly". (You can see this kind of thing if
you valgrind a TCG QEMU, for instance, I think.)

As you say, icount is supposed to be the other option. I've
never looked closely enough at it to figure out exactly
how it works...

-- PMM