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

[Alexander Graf <agraf@csgraf.de>]

On 03.12.20 00:28, Peter Collingbourne wrote:
> On Wed, Dec 2, 2020 at 2:57 PM Alexander Graf <agraf@csgraf.de> wrote:
>>
>> On 02.12.20 23:46, Frank Yang wrote:
>>
>>
>>
>> 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
>>>
>>> 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?
>>
>>
>> So QEMU_CLOCK_VIRTUAL calls cpu_get_clock() which just adds an offset to gettimeofday(). This offset is already part of the live migration stream[1]. So if you just configure CNTVOFF_EL2 based on QEMU_CLOCK_VIRTUAL adjusted by the clock frequency on vcpu init, you should have everything you need. You can do that on every CPU init even, as the virtual clock will just be 0 on start.
>>
>> The only thing we need to change then is to move the WFI from a direct call to mach_absolute_time() to also check the virtual clock instead. I would hope that gettimeofday() calls mach_absolute_time() in the background too to speed it up.
> I'm not sure that something based on gettimeofday() (or
> clock_gettime(CLOCK_MONOTONIC) which it looks like cpu_get_clock() can
> also call) will work. It will include time spent asleep so it won't
> correspond to mach_absolute_time() aka guest CNTVCT_EL0.


It will correspond to it on launch, because we'd set the offset. 
Afterwards, the only problem I think you're unraveling here is that we 
would need to adjust CNTVOFF after a suspend/resume cycle to propagate 
the time jump into the virtual machine. Which probably is not a terrible 
idea anyway, because how else would the guest know that time has passed?


Alex