Re: [PATCH v1 1/1] hvf: arm: Properly sync guest time on migration

[Frank Yang]

On Wed, Dec 2, 2020 at 2:28 PM Alexander Graf <agraf@csgraf.de> wrote:

On 02.12.20 23:19, Frank Yang wrote:

From downstream: https://android-review.googlesource.com/c/platform/external/qemu/+/1515002

Based on v3 of Alexander Graf's patches

20201202190408.2041-1-agraf@csgraf.de" target="_blank">https://patchew.org/QEMU/20201202190408.2041-1-agraf@csgraf.de

We need to adjust CNTVOFF_EL2 so that time doesnt warp.  Even though we
can set separate CNTVOFF_EL2 values per vCPU, it just is not worth the
require effort to do that accurately---with individual values, even if
they are a tiny bit off it can result in a lockup due to inconsistent
time differences between vCPUs. So just use a global approximate value
for now.

Not tested in upstream yet, but Android emulator snapshots work without
time warp now.

Signed-off-by: Lingfeng Yang <lfy@google.com>

If we just always make CNTV start at the same 0 as QEMU_CLOCK_VIRTUAL, we should be able to just recover the offset after migration by looking at QEMU_CLOCK_VIRTUAL to set CNTVOFF, right?

That would end up much easier than this patch I hope.

The virtual clock interfaces/implementations in QEMU seem complex to me relative to the fix needed here and they don't seem to compute ticks with mach_absolute_time() (which in this case we want since we want to compute in timer ticks instead of having to mess with ns / cycle conversions). I do agree this patch does seem more complicated on the surface though versus "just" setting cntvoff directly to some value. Maybe we should simplify the QEMU_CLOCK_VIRTUAL implementation first to maintain CNTVOFF_EL2/CNTV using mach_absolute_time() first?

Alex