On 01/08/2017 15:36, Brijesh Singh wrote: >> >> The flow is: >> >> hardware walks page table; L2 page table points to read only memory >> -> pf_interception (code = >> -> kvm_handle_page_fault (need_unprotect = false) >> -> kvm_mmu_page_fault >> -> paging64_page_fault (for example) >> -> try_async_pf >> map_writable set to false >> -> paging64_fetch(write_fault = true, map_writable = false, >> prefault = false) >> -> mmu_set_spte(speculative = false, host_writable = false, >> write_fault = true) >> -> set_spte >> mmu_need_write_protect returns true >> return true >> write_fault == true -> set emulate = true >> return true >> return true >> return true >> emulate >> >> Without this patch, emulation would have called >> >> ..._gva_to_gpa_nested >> -> translate_nested_gpa >> -> paging64_gva_to_gpa >> -> paging64_walk_addr >> -> paging64_walk_addr_generic >> set fault (nested_page_fault=true) >> >> and then: >> >> kvm_propagate_fault >> -> nested_svm_inject_npf_exit >> > > maybe then safer thing would be to qualify the new error_code check with > !mmu_is_nested(vcpu) or something like that. So that way it would run on > L1 guest, and not the L2 guest. I believe that would restrict it avoid > hitting this case. Are you okay with this change ? Or check "vcpu->arch.mmu.direct_map"? That would be true when not using shadow pages. > IIRC, the main place where this check was valuable was when L1 guest had > a fault (when coming out of the L2 guest) and emulation was not needed. How do I measure the effect? I tried counting the number of emulations, and any difference from the patch was lost in noise. Paolo
