On Sun, Jul 5, 2020 at 7:12 PM Nadav Amit <nadav.amit@xxxxxxxxx> wrote: > > > On Jul 4, 2020, at 11:38 PM, Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote: > > > > On 04/07/20 02:00, Nadav Amit wrote: > >> Hello Paolo, > >> > >> I encountered an issue while running some svm tests. Apparently, the tests > >> “npt_rw_pfwalk” and “npt_rsv_pfwalk” expect the present bit to be clear. > >> > >> KVM indeed clears this bit in nested_svm_inject_npf_exit(): > >> > >> /* > >> * The present bit is always zero for page structure faults on real > >> * hardware. > >> */ > >> if (svm->vmcb->control.exit_info_1 & (2ULL << 32)) > >> svm->vmcb->control.exit_info_1 &= ~1; > >> > >> > >> I could not find documentation of this behavior. Unfortunately, I do not > >> have a bare-metal AMD machine to test the behavior (and some enabling of > >> kvm-unit-tests/svm is required, e.g. this test does not run with more than > >> 4GB of memory). > >> > >> Are you sure that this is the way AMD machines behave? > > > > No, I'm not. The code was added when NPF was changed to synthesize > > EXITINFO1, instead of simply propagating L0's EXITINFO1 into L1 (see > > commit 5e3525195196, "KVM: nSVM: propagate the NPF EXITINFO to the > > guest", 2014-09-03). With six more years of understanding of KVM, the > > lack of a present bit might well have been a consequence of how the MMU > > works. > > Thanks. I ran ‘git blame’ before asking you, and that is the reason I > assumed you would know best... ;-) > > > One of these days I'd like to run the SVM tests under QEMU without KVM. > > It would probably find bugs in both. > > Well, I think we can agree that bare-metal is a better reference than > another emulator for the matter. Even without running the tests on > bare-metal, it is easy to dump EXITINFO1 on the nested page-fault. I will > try to find a bare-metal machine. > > Anyhow, I would appreciate if anyone from AMD would tell whether any result > should be considered architectural. I'd be happy to test on bare metal, but I'm still waiting for instructions that a script kiddie (with read-only console access) can follow.
