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, 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 _______________________________________________ devel mailing list devel@xxxxxxxxxxxxxxxxxxxxxx http://driverdev.linuxdriverproject.org/mailman/listinfo/driverdev-devel
