On Thu, Nov 17, 2022, Oliver Upton wrote: > On Thu, Nov 17, 2022 at 12:23:50AM +0000, Sean Christopherson wrote: > > Automatically disable single-step when the guest reaches the end of the > > verified section instead of using an explicit ucall() to ask userspace to > > disable single-step. An upcoming change to implement a pool-based scheme > > for ucall() will add an atomic operation (bit test and set) in the guest > > ucall code, and if the compiler generate "old school" atomics, e.g. > > Off topic, but I didn't ask when we were discussing this issue. What is > the atomic used for in the pool-based ucall implementation? To avoid having to plumb an "id" into the guest, vCPUs grab a ucall entry from the pool on a first-come first-serve basis, and then release the entry when the ucall is complete. The current implementation is a bitmap, e.g. every possible entry has a bit in the map, and vCPUs do an atomic bit-test-and-set to claim an entry. Ugh. And there's a bug. Of course I notice it after sending the pull request. Depsite being defined in atomic.h, and despite clear_bit() being atomic in the kernel, tools' clear_bit() isn't actually atomic. Grr. Doesn't cause problems because there are so few multi-vCPU selftests, but that needs to be fixed. Best thing would be to fix clear_bit() itself. _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
