Il mer 22 gen 2025, 07:07 John Stultz <jstultz@xxxxxxxxxx> ha scritto: > > I then cut down and ported the bionic test out so it could build under > a standard debian environment: > https://github.com/johnstultz-work/bionic-ptrace-reproducer > > Where I was able to reproduce the same problem in a debian VM (after > running the test in a loop for a short while). Thanks, that's nice to have. > Now, here's where it is odd. I could *not* reproduce the problem on > bare metal hardware, *nor* could I reproduce the problem in a virtual > environment. I can *only* reproduce the problem with nested > virtualization (running the VM inside a VM). Typically in that case the best thing to do is turn it into a kvm-unit-test or selftest (though that's often an endeavor of its own, as it requires distilling the Linux kernel and userspace code into a guest that runs without an OS). But what you've done is already a good step. > I have reproduced this on my intel i12 NUC using the same v6.12 kernel > on metal + virt + nested environments. It also reproduced on the NUC > with v5.15 (metal) + v6.1 (virt) + v6.1(nested). Good that you can use a new kernel. Older kernels are less reliable with nested virt (especially since the one that matters the most is the metal one). Paolo > I've tried to do some tracing in the arch/x86/kvm/x86.c logic, but > I've not yet directly correlated anything on the hosts to the point > where we read the zero DR6 value in the nested guest. > > But I'm not very savvy around virtualization or ptrace watchpoints or > low level details around intel DB6 register, so I wanted to bring this > up on the list to see if folks had suggestions or ideas to further > narrow this down? Happy to test things as it's pretty simple to > reproduce here. > > Many thanks to Alex Bennee and Jim Mattson for their testing > suggestions to help narrow this down so far. > > thanks > -john >
