Re: qemu polling KVM_IRQ_LINE_STATUS when stopped

[Kevin Locke <kevin@kevinlocke.name>]

Thanks for the info Paolo and Andi!

On Thu, 2020-06-25 at 20:41 +0200, Paolo Bonzini wrote:
> On 25/06/20 16:26, Kevin Locke wrote:
>> 1. Do I understand correctly that the CPU usage is due to counting
>>    RTC periodic timer ticks for replay when the guest is resumed?
> 
> Yes.
> 
>> 2. If so, would it be possible to calculate the number of ticks
>>    required from the time delta at resume, rather than polling each
>>    tick while paused?
> 
> Note that high CPU usage while the guest is paused is a bug.  Only high
> CPU usage as soon as the guest resumes is the unavoidable part.
 
[...]
 
>> 5. I have not observed high CPU usage for paused VMs in VirtualBox.
>>    Would it be worth investigating how they handle this?
>> 
>> From the discussion in https://bugs.launchpad.net/bugs/1851062 it
>> appears that the issue does not occur for all Windows 10 VMs.  Does
>> that fit the theory it is caused by RTC periodic timer ticks?
> 
> Windows 10 can use the Hyper-V synthetic timer instead of the RTC, which
> shouldn't have the problem.

That's great news!  Since I'm able to reproduce the issue on a
recently installed Windows 10 2004 VM on Linux 5.7 with QEMU 5.0, is
there anything I can do to help isolate the bug, or is it a known
issue?

Thanks again,
Kevin