High cpu usage when guest is idle on qemu-system-i386

[John Arbuckle]

When running Windows XP in qemu-system-i386, the cpu usage of QEMU is about 100% even when the guest CPU usage is close to 2%. The host cpu usage should be low when the guest cpu usage is low.

Command: qemu-system-i386 -hda <Windows XP HD image>

Using this command also shows around 100% host CPU usage:
qemu-system-i386 -m 700 -hda <Windows XP HD image> -usb -device usb-audio -net nic,model=rtl8139 -net user -hdb mountable.img -soundhw pcspk

Using the Penryn CPU option also saw this problem:
qemu-system-i386 -hda <Windows XP HD image> -m 700 -cpu Penryn-v1

Using "-cpu pentium2" saw the same high host cpu usage.

My Info:
M1 MacBook Air
Mac OS 11.1
qemu-system-i386 version 5.2 (1ba089f2255bfdb071be3ce6ac6c3069e8012179)
Windows XP SP3 Build 2600

[John Arbuckle (programmingkidx)]

Just to compare notes I ran my same Windows XP image on an older version of QEMU. This is version 2.10.1. It was built for the x86_64 architecture. The host CPU architecture is aarm64. The host CPU usage was actually very low when the guest CPU usage was low. The guest was using about 8% and the host usage was around 14%.

[John Arbuckle (programmingkidx)]

For version 5.2 of qemu-system-i386 the instruction the guest is busy executing over and over again is this: addb %al, (%eax)

For version 2.10.1 this is the instruction that is being executed when the guest is idle:
add %al,(%eax)

[John Arbuckle (programmingkidx)]

After updating QEMU to 1214d55d1c41fbab3a9973a05085b8760647e411, I reinstalled Windows XP and the host CPU usage at idle was normal. My guess is that I picked a bad commit to reinstall Windows XP.

I tried using "-smp 4". Windows XP started up to a black screen. When I restarted the problem with high CPU usage at idle was back. I did not use the "-smp 4" option after restarting.

When I first specified the '-smp 4' option I saw Windows install something then have the computer restarted.

[John Arbuckle (programmingkidx)]

I found a way to fix the high host cpu usage issue. To fix this issue click on Start->All Programs->Accessories->System Tools->System Restore. Then pick a restore point that is set before you tried the smp option. After the VM restarts the high CPU usage issue will be gone :)

[Thomas Huth (th-huth)]

The QEMU project is currently moving its bug tracking to another system.
For this we need to know which bugs are still valid and which could be
closed already. Thus we are setting the bug state to "Incomplete" now.

If the bug has already been fixed in the latest upstream version of QEMU,
then please close this ticket as "Fix released".

If it is not fixed yet and you think that this bug report here is still
valid, then you have two options:

1) If you already have an account on gitlab.com, please open a new ticket
for this problem in our new tracker here:

    https://gitlab.com/qemu-project/qemu/-/issues

and then close this ticket here on Launchpad (or let it expire auto-
matically after 60 days). Please mention the URL of this bug ticket on
Launchpad in the new ticket on GitLab.

2) If you don't have an account on gitlab.com and don't intend to get
one, but still would like to keep this ticket opened, then please switch
the state back to "New" or "Confirmed" within the next 60 days (other-
wise it will get closed as "Expired"). We will then eventually migrate
the ticket automatically to the new system (but you won't be the reporter
of the bug in the new system and thus you won't get notified on changes
anymore).

Thank you and sorry for the inconvenience.

[Launchpad Janitor (janitor)]

[Expired for QEMU because there has been no activity for 60 days.]