Re: [PATCH v4 00/10] make L2's kvm-clock stable, get rid of pvclock_gtod_copy in KVM

[Denis Plotnikov <dplotnikov@virtuozzo.com>]

ping!

On 02.08.2017 20:11, Paolo Bonzini wrote:
> On 02/08/2017 18:49, John Stultz wrote:
>> On Wed, Aug 2, 2017 at 7:38 AM, Denis Plotnikov
>> <dplotnikov@virtuozzo.com> wrote:
>>> V4:
>>>    * removed "is stable" function with vague definition of stability
>>>      there is the only function which does time with cycle stamp getting
>>>    * some variables renamed
>>>    * some patches split into smaller once
>>>    * atomic64_t usage is replaced with atomic_t
>>>
>>> V3:
>>>    Changing the timekeeper interface for clocksource reading looks like
>>>    an overkill to achive the goal of getting cycles stamp for KVM.
>>>    Instead extend the timekeeping interface and add functions which provide
>>>    necessary data: read clocksource with cycles stamp, check whether the
>>>    clock source is stable.
>>>
>>>    Use those functions and improve existing timekeeper functionality to
>>>    replace pvclock_gtod_copy scheme in masterclock data calculation.
>>>
>>> V2:
>>>    The main goal is to make L2 kvm-clock be stable when it's running over L1
>>>    with stable kvm-clock.
>>>
>>>    The patch series is for x86 architecture only. If the series is approved
>>>    I'll do changes for other architectures but I don't have an ability to
>>>    compile and check for every single on (help needed)
>>>
>>>    The patch series do the following:
>>>
>>>          * change timekeeper interface to get cycles stamp value from
>>>            the timekeeper
>>>          * get rid of pvclock copy in KVM by using the changed timekeeper
>>>            interface: get time and cycles right from the timekeeper
>>>          * make KVM recognize a stable kvm-clock as stable clocksource
>>>            and use the KVM masterclock in this case, which means making
>>>            L2 stable when running over stable L1 kvm-clock
>>
>> So, from a brief skim, I'm not a big fan of this patchset. Though this
>> is likely in part due to that I haven't seen anything about *why*
>> these changes are needed.
> 
>  From my selfish KVM maintainer point of view, one advantage is that it
> drops knowledge of internal timekeeping functioning from KVM, using
> ktime_get_snapshot instead.  These are patches 1-5.  Structuring the
> series like this was my idea so I take the blame.
> 
> As to patches 6-10, KVM is currently only able to provide vsyscalls if
> the host is using the TSC.  However, when using nested virtualization
> you have
> 
> 	L0: bare-metal hypervisor (uses TSC)
> 	L1: nested hypervisor (uses kvmclock, can use vsyscall)
> 	L2: nested guest
> 
> and L2 cannot use vsyscall because it is not using the TSC.  This series
> lets you use the vsyscall in L2 as long as L1 can.
> 
> There is one point where I couldn't help Denis as much as I wanted.
> That's a definition of what's a "good" clocksource that can be used by
> KVM to provide the vsyscall.  I know why the patch is correct, but I
> couldn't really define the concept.
> 
> In ktime_get_snapshot and struct system_counterval_t's users, they seem
> to use "cycles" to map from TSC to ART; this is not unlike kvmclock's
> use of "cycles" to map from TSC to nanoseconds at an origin point.
> However, it's not clear to me whether "cycles" may be used by
> adjust_historical_crosststamp even for non-TSC clocksources (or
> non-kvmclock after this series).  It doesn't help that
> adjust_historical_crosststamp is essentially dead code, since
> get_device_system_crosststamp is always called with a NULL history argument.
> 
> I'm also CCing Marcelo who wrote the KVM vsyscall code.
> 
> Paolo
> 
>> Can you briefly explain the issue you're trying to solve, and why you
>> think this approach is the way to go?
>> (Its usually a good idea to have such rational included in the patchset)
> 

-- 
Best,
Denis