Re: qemu polling KVM_IRQ_LINE_STATUS when stopped

[Kevin Locke <kevin@kevinlocke.name>]

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.

Although enabling the hv_stimer Hyper-V enlightenment avoids the
issue, I assume high CPU usage while the guest is paused when
hv_stimer is not enabled is still a bug.  It's not important for my
current use cases, but if there is interest in fixing the issue for
others, I was able to find a more minimal reproduction:

Using the Windows 10 May 2020 English 64-bit ISO from
https://www.microsoft.com/en-us/software-download/windows10ISO

If I run

qemu-system-x86_64 \
	-no-user-config \
	-machine pc-q35-5.0,accel=kvm \
	-m 1024 \
	-blockdev driver=file,node-name=win10iso,filename=Win10_2004_English_x64.iso \
	-device ide-cd,drive=win10iso \
	-no-hpet \
	-rtc driftfix=slew

then pause the VM after the "Windows Setup" window appears,
qemu-system-x86_64 uses ~40% CPU indefinitely.  Without
-rtc driftfix=slew, ~10%.  Without -no-hpet, ~1%.

For reference, both of these options are added by virt-manager by
default for Windows 10.

If there's anything else I can do to help, let me know.
Thanks again for all of the help tracking this down.

Best,
Kevin