Re: [patch 00/11] x86/vdso: Cleanups, simmplifications and CLOCK_TAI support

[Marcelo Tosatti]

On Wed, Oct 03, 2018 at 03:32:08PM -0700, Andy Lutomirski wrote:
> On Wed, Oct 3, 2018 at 12:01 PM Marcelo Tosatti <mtosatti@xxxxxxxxxx> wrote:
> >
> > On Tue, Oct 02, 2018 at 10:15:49PM -0700, Andy Lutomirski wrote:
> > > Hi Vitaly, Paolo, Radim, etc.,
> > >
> > > On Fri, Sep 14, 2018 at 5:52 AM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> > > >
> > > > Matt attempted to add CLOCK_TAI support to the VDSO clock_gettime()
> > > > implementation, which extended the clockid switch case and added yet
> > > > another slightly different copy of the same code.
> > > >
> > > > Especially the extended switch case is problematic as the compiler tends to
> > > > generate a jump table which then requires to use retpolines. If jump tables
> > > > are disabled it adds yet another conditional to the existing maze.
> > > >
> > > > This series takes a different approach by consolidating the almost
> > > > identical functions into one implementation for high resolution clocks and
> > > > one for the coarse grained clock ids by storing the base data for each
> > > > clock id in an array which is indexed by the clock id.
> > > >
> > >
> > > I was trying to understand more of the implications of this patch
> > > series, and I was again reminded that there is an entire extra copy of
> > > the vclock reading code in arch/x86/kvm/x86.c.  And the purpose of
> > > that code is very, very opaque.
> > >
> > > Can one of you explain what the code is even doing?  From a couple of
> > > attempts to read through it, it's a whole bunch of
> > > probably-extremely-buggy code that,
> >
> > Yes, probably.
> >
> > > drumroll please, tries to atomically read the TSC value and the time.  And decide whether the
> > > result is "based on the TSC".
> >
> > I think "based on the TSC" refers to whether TSC clocksource is being
> > used.
> >
> > > And then synthesizes a TSC-to-ns
> > > multiplier and shift, based on *something other than the actual
> > > multiply and shift used*.
> > >
> > > IOW, unless I'm totally misunderstanding it, the code digs into the
> > > private arch clocksource data intended for the vDSO, uses a poorly
> > > maintained copy of the vDSO code to read the time (instead of doing
> > > the sane thing and using the kernel interfaces for this), and
> > > propagates a totally made up copy to the guest.
> >
> > I posted kernel interfaces for this, and it was suggested to
> > instead write a "in-kernel user of pvclock data".
> >
> > If you can get kernel interfaces to replace that, go for it. I prefer
> > kernel interfaces as well.
> >
> > >  And gets it entirely
> > > wrong when doing nested virt, since, unless there's some secret in
> > > this maze, it doesn't acutlaly use the scaling factor from the host
> > > when it tells the guest what to do.
> > >
> > > I am really, seriously tempted to send a patch to simply delete all
> > > this code.
> >
> > If your patch which deletes the code gets the necessary features right,
> > sure, go for it.
> >
> > > The correct way to do it is to hook
> >
> > Can you expand on the correct way to do it?
> >
> > > And I don't see how it's even possible to pass kvmclock correctly to
> > > the L2 guest when L0 is hyperv.  KVM could pass *hyperv's* clock, but
> > > L1 isn't notified when the data structure changes, so how the heck is
> > > it supposed to update the kvmclock structure?
> >
> > I don't parse your question.
> 
> Let me ask it more intelligently: when the "reenlightenment" IRQ
> happens, what tells KVM to do its own update for its guests?

Update of what, and why it needs to update anything from IRQ? 

The update i can think of is from host kernel clocksource, 
which there is a notifier for.