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

[Andy Lutomirski]

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?