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

    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.

    >
    > --Andy

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