Brijesh, just one thing I'd like to double check: does the erratum really depend on guest CR4.SMAP, or rather on host CR4.SMAP? (Of course the SMEP check instead needs to use guest CR4.SMEP). And here is a commit message with style fixes: --- When CPU raise #NPF on guest data access and guest CR4.SMAP=1, it is possible that CPU microcode implementing DecodeAssist will fail to read bytes of instruction which caused #NPF. This is AMD errata 1096 and it happens because CPU microcode reading instruction bytes incorrectly attempts to read code as implicit supervisor-mode data accesses (that is, just like it would read e.g. a TSS), which are susceptible to SMAP faults. The microcode reads CS:RIP and if it is a user-mode address according to the page tables, the processor gives up and returns no instruction bytes. In this case, GuestIntrBytes field of the VMCB on a VMEXIT will incorrectly return 0 instead of the correct guest instruction bytes. Current KVM code attemps to detect and workaround this errata, but it has multiple issues: 1) It mistakenly checks if guest CR4.SMAP=0 instead of guest CR4.SMAP=1, which is required for encountering a SMAP fault. 2) It assumes SMAP faults can only occur when vCPU CPL==3. However, in case vCPU CR4.SMEP=0, the guest can execute an instruction which reside in a user-accessible page with CPL<3 priviledge. If this instruction raise a #NPF on it's data access, then CPU DecodeAssist microcode will still encounter a SMAP violation. Even though no sane OS will do so (as it's an obvious priviledge escalation vulnerability), we still need to handle this semanticly correct in KVM side. Note that (2) *is* a useful optimization, because CR4.SMAP=1 is an easy triggerable condition and guests usually enable SMAP together with SMEP, If the vCPU has CR4.SMEP=1, the errata could indeed be encountered only at guest CPL3; otherwise, the CPU would raise a SMEP fault to guest instead of #NPF. We keep this condition to avoid false positives in the detection of the errata. In addition, to avoid future confusion and improve code readbility, include details of the errata in code and not just in commit message. --- Paolo
