Re: [9front] qemu clock/timer issue

[Jacob Moody <moody@posixcafe.org>]

On 4/16/24 13:46, Michael Forney wrote:
> Jacob Moody <moody@posixcafe.org> wrote:
>> It is understood that this bug only happens with intel kvm virtual machines.
>>  From all that has been tested it was currently understood to affect newer intel machines.
>> It seems however that your machine is a bit older(10+ years) and still triggering this.
>> I've certainly run 9front in kvm on intel machines before so it is likely either
>> something that changed with us or something that has changed with QEMU. My money
>> is on some QEMU change.
>>
>> If you do not feel comfortable debugging but would still be interested in helping
>> perhaps you can help bisect 9front and qemu (one side at a time) to see if you can
>> narrow down what has changed to get more information to fix these things.
>> I would start with the nightly iso just to double check, then move back through the releases
>> going back. Give that a go for a couple of them, then maybe try some older qemu.
> 
> If you look at my original mails, I had already bisected it to:
> http://git.9front.org/plan9front/plan9front/a05bab362f66ddd6fa65f2e7cda9eaaa0217ec08/commit.html
> 
> This changes the mechanism by which CPU frequencies are detected
> by default in 9front, and the timing/clock issues arise when the
> frequency is detected incorrectly as 25MHz instead of 3.3GHz. The
> issue went away when using *nohpet=1 to revert to i8253
> clockinit/fastclock.
> 
> I wasn't able to explain why hpetcpufreq didn't work, but i8253cpufreq
> did. They are both emulated with QEMU_CLOCK_VIRTUAL:
> https://github.com/qemu/qemu/blob/62dbe54c24dbf77051bafe1039c31ddc8f37602d/hw/timer/hpet.c#L156
> https://github.com/qemu/qemu/blob/62dbe54c24dbf77051bafe1039c31ddc8f37602d/hw/timer/i8254.c#L57-L58

Ah my bad, I had forgotten about this. Thank you for the original bisecting work and for reiterating.

- moody