On Thu, Nov 17, 2022, Sean Christopherson wrote: > 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. Ha! And I bet when clear_bit() is fixed, this test will start failing again because the ucall() to activate single-step needs to release the entry _after_ exiting to the host, i.e. single-step will be enabled across the atomic region again. _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
