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. One of these days I'd like to run the SVM tests under QEMU without KVM. It would probably find bugs in both. Paolo
