On 05/11/19 14:54, Andrea Arcangeli wrote: > x86-64 is already bisectable. > > All other archs bisectable I didn't check them all anyway. > > Even 4/13 is suboptimal and needs to be re-done later in more optimal > way. I prefer all logic changes to happen at later steps so one can at > least bisect to something that functionally works like before. And > 4/13 also would need to be merged in the huge patch if one wants to > guarantee bisectability on all CPUs, but it'll just be hidden there in > the huge patch. > > Obviously I can squash both 3/13 and 4/13 into 2/13 but I don't feel > like doing the right thing by squashing them just to increase > bisectability. You can reorder patches so that kvm_x86_ops assignments never happen. That way, 4/13 for example would be moved to the very beginning. >> - look into how to remove the modpost warnings. A simple (though >> somewhat ugly) way is to keep a kvm.ko module that includes common >> virt/kvm/ code as well as, for x86 only, page_track.o. A few functions, >> such as kvm_mmu_gfn_disallow_lpage and kvm_mmu_gfn_allow_lpage, would >> have to be moved into mmu.h, but that's not a big deal. > > I think we should: > > 1) whitelist to shut off the warnings on demand Do you mean adding a whitelist to modpost? That would work, though I am not sure if the module maintainer (Jessica Yu) would accept that. > 2) verify that if two modules are registering the same export symbol > the second one fails to load and the module code is robust about > that, this hopefully should already be the case > > Provided verification of 2), the whitelist is more efficient than > losing 4k of ram in all KVM hypervisors out there. I agree. >> - provide at least some examples of replacing the NULL kvm_x86_ops >> checks with error codes in the function (or just early "return"s). I >> can help with the others, but remember that for the patch to be merged, >> kvm_x86_ops must be removed completely. > > Even if kvm_x86_ops wouldn't be guaranteed to go away, this would > already provide all the performance benefit to the KVM users, so I > wouldn't see a reason not to apply it even if kvm_x86_ops cannot go > away. The answer is maintainability. My suggestion is that we start looking into removing all assignments and tests of kvm_x86_ops, one step at a time. Until this is done, unfortunately we won't be able to reap the performance benefit. But the advantage is that this can be done in many separate submissions; it doesn't have to be one huge patch. Once this is done, removing kvm_x86_ops is trivial in the end. It's okay if the intermediate step has minimal performance regressions, we know what it will look like. I have to order patches with maintenance first and performance second, if possible. By the way, we are already planning to make some module parameters per-VM instead of global, so this refactoring would also help that effort. > Said that it will go away and there's no concern about it. It's > just that the patchset seems large enough already and it rejects > heavily already at every port. I simply stopped at the first self > contained step that provides all performance benefits. That is good enough to prove the feasibility of the idea, so I agree that was a good plan. Paolo > If I go ahead and remove kvm_x86_ops how do I know it won't reject > heavily the next day I rebase and I've to redo it all from scratch? If > you explain me how you're going to guarantee that I won't have to do > that work more than once I'd be happy to go ahead.
