Re: Recording and replaying HPET timer values

[Pavel Dovgalyuk]

On 14.05.2021 06:30, Arnabjyoti Kalita wrote:
> Hello all,
>
> I am recording and replaying HPET timer values for my guest.
>
> I am running QEMU version 5.0.1. The guest kernel is a
> 4.4.0-21-generic Ubuntu flavor. The host and target architecture is
> both x86-64.
>
> To record, I start QEMU in KVM mode as follows -
>
> sudo ./qemu-system-x86_64 -m 1024 --machine pc-i440fx-2.5 -cpu
> qemu64,-kvmclock -enable-kvm -clock_replay
> mode=record,file=clock_record9.txt -netdev
> tap,id=tap1,ifname=tap0,script=no,downscript=no -device
> virtio-net-pci,netdev=tap1,mac=00:00:00:00:00:00 -drive
> file=~/os_images_for_qemu/ubuntu-16.04-desktop-amd64.qcow2,format=qcow2,if=none,id=img-direct
> -device virtio-blk-pci,drive=img-direct
>
> After this, I force the guest to switch to the "hpet" clocksource.
>
> The clock_record9.txt file records the values of the HPET counter. To
> perform the recording, I have changed the hpet_ram_read function in
> the "hw/timer/hpet.c" file.
>
> static uint64_t hpet_ram_read() {
> .....
>      case HPET_COUNTER:
>          if (hpet_enabled(s)) {
>              cur_tick = hpet_get_ticks(s);
>          } else {
>              cur_tick = s->hpet_counter;
>          }
>          record_hpet_counter((int64_t)cur_tick);  <-----  record here
>          return cur_tick;
> }
>
> In replay mode, I start the guest in TCG mode as below-
>
> sudo ./qemu-system-x86_64 -m 1024 -clock_replay
> mode=replay,file=clock_record9.txt -machine pc-i440fx-2.5 -cpu qemu64
> -netdev tap,id=tap1,ifname=tap0,script=no,downscript=no -device
> virtio-net-pci,netdev=tap1,mac=00:00:00:00:00:00 -drive
> file=~/os_images_for_qemu/ubuntu-16.04-desktop-amd64.qcow2,format=qcow2,if=none,id=img-direct
> -device virtio-blk-pci,drive=img-direct -loadvm snapshot1
>
> This is supposed to replay and mimic the HPET counter values. The
> snapshot is there to ensure that both record and replay start from the
> same guest state.
>
> Is the guest replay going to be deterministic, with respect to the
> HPET clock? Are there other factors about the HPET clock that need to
> be taken into account while replaying it ?

I don't sure that this is enough.
There is qemu_timer in hpet.c, which is used for injecting the interrupts.
And interrupt handler code may read HPET clock. I mean that there may be 
different frequency or timer reads in different runs.

> I am not worried about other I/O events right now and I just want to
> ensure that HPET timer values are being replayed correctly.
>
> Best Regards,
> Arnabjyoti Kalita