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