On Aug 17, 2014, at 1:21 AM, Paolo Bonzini wrote: > Il 15/08/2014 23:49, Hulin, Patrick - 0559 - MITLL ha scritto: >>>> In this case, the write is 8 bytes and unaligned, so it gets split >>>> into 8 single-byte writes. In stock QEMU, these writes are done in >>>> reverse order (see the loop in softmmu_template.h, line 402). The >>>> third decryption xor from Kernel Patch Protection should hit 4 bytes >>>> that are in the current TB and 4 bytes in the TB afterwards in linear >>>> order. Since this happens in reverse order, and the last 4 bytes of >>>> the write do not intersect the current TB, those writes happen >>>> successfully and QEMU's memory is modified. The 4th byte in linear >>>> order (the 5th in temporal order) then triggers the >>>> current_tb_modified flag and cpu_restore_state, longjmp'ing out. >>>> >>> Would it work to just call tb_invalidate_phys_page_range before the >>> helper_ret_stb loop? >> >> Maybe. I think there’s another issue, which is that QEMU’s ending up >> in the I/O read/write code instead of the normal memory RW. This could >> be QEMU messing up, it could be PatchGuard doing something weird, or it >> could be me misunderstanding what’s going on. I never really figured out >> how the control flow works here. > > That's okay. Everything that's in the slow path goes down > io_mem_read/write (in this case TLB_NOTDIRTY is set for dirty-page > tracking and causes QEMU to choose that path). > > Try making a self-contained test case using the kvm-unit-tests harness > (git://git.kernel.org/pub/scm/virt/kvm/kvm-unit-tests.git). > > Paolo Okay. I’ve attached a test case. Note that since this has to run without KVM it messes around with the run script.