Hi the KVM community, I am sending this email to ask about how the KVM emulates the effect of guest MTRRs on AMD platforms. Since there is no hardware support for guest MTRRs, the VMM can simulate their effect by altering the memory types in the EPT/NPT. From my understanding, this is exactly what the KVM does for Intel platforms. More specifically, in arch/x86/kvm/mmu/spte.c #make_spte(), the KVM tries to respect the guest MTRRs by calling #kvm_x86_ops.get_mt_mask() to get the memory types indicated by the guest MTRRs and applying that to the EPT. For Intel platforms, the implementation of #kvm_x86_ops.get_mt_mask() is #vmx_get_mt_mask(), which calls the #kvm_mtrr_get_guest_memory_type() to get the memory types indicated by the guest MTRRs. However, on AMD platforms, the KVM does not implement #kvm_x86_ops.get_mt_mask() at all, so it just returns zero. Does it mean that the KVM does not use the NPT to emulate the effect of guest MTRRs on AMD platforms? I tried but failed to find out how the KVM does for AMD platforms. Can someone help me understand how the KVM emulates the effect of guest MTRRs on AMD platforms? Thanks a lot! Best, Yibo
