On Wed, Dec 2, 2020 at 2:28 PM Alexander Graf <ag...@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 > > https://patchew.org/QEMU/20201202190408.2041-1-ag...@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 <l...@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 > > >
