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

Andy Lutomirski
Wed, 03 Oct 2018 05:15:49 -0000

Hi Vitaly, Paolo, Radim, etc.,

On Fri, Sep 14, 2018 at 5:52 AM Thomas Gleixner <tglx at linutronix.de>
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, drumroll please, tries to
atomically read the TSC value and the time.  And decide whether the
result is "based on the TSC".  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.  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.  The correct way to do it is to hook

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?

--Andy