To: [email protected] Cc: Paolo Bonzini <[email protected]>, Marcelo Tosatti <[email protected]>, [email protected]
KVM normally recognizes ICEBP, advances RIP, and injects #DB itself. When userspace enables hardware breakpoints, however, KVM reports the pre-skip #DB through KVM_EXIT_DEBUG. QEMU currently reinjects it at the old RIP, causing a debug-exit livelock. The patch is marked RFC because KVM_EXIT_DEBUG does not preserve KVM's exact ICEBP classification. The proposed compatibility heuristic accepts a causeless #DB whose instruction stream consists only of prefixes and 0xf1. A durable long-term ABI might instead let KVM expose its exact classification or advance the instruction before exiting; I would like feedback from both QEMU and KVM reviewers on that direction. Validation on an Intel VT-x host: - Built x86_64-softmmu from current master (ae4f3443209). - checkpatch reports zero errors and warnings. - Ran a real Linux guest that executes kernel-mode ICEBP while an unrelated hardware breakpoint is installed through QEMU's GDB stub. Unpatched QEMU stalls after ICEBP_KERNEL_BEFORE; patched QEMU reaches ICEBP_KERNEL_AFTER. I did not include the hardware reproduction in the patch because it needs real VT-x, a matching guest kernel build, and a loadable module. I can turn the fixture into an optional avocado test if reviewers prefer. Saul Freedman (1): target/i386: skip ICEBP before reinjecting #DB target/i386/kvm/kvm.c | 62 ++++++++++++++++++++++++++++++++++++++++++- 1 file changed, 61 insertions(+), 1 deletion(-) -- 2.55.0
