1.. SPDX-License-Identifier: GPL-2.0 2 3========== 4Nested VMX 5========== 6 7Overview 8--------- 9 10On Intel processors, KVM uses Intel's VMX (Virtual-Machine eXtensions) 11to easily and efficiently run guest operating systems. Normally, these guests 12*cannot* themselves be hypervisors running their own guests, because in VMX, 13guests cannot use VMX instructions. 14 15The "Nested VMX" feature adds this missing capability - of running guest 16hypervisors (which use VMX) with their own nested guests. It does so by 17allowing a guest to use VMX instructions, and correctly and efficiently 18emulating them using the single level of VMX available in the hardware. 19 20We describe in much greater detail the theory behind the nested VMX feature, 21its implementation and its performance characteristics, in the OSDI 2010 paper 22"The Turtles Project: Design and Implementation of Nested Virtualization", 23available at: 24 25 https://www.usenix.org/events/osdi10/tech/full_papers/Ben-Yehuda.pdf 26 27 28Terminology 29----------- 30 31Single-level virtualization has two levels - the host (KVM) and the guests. 32In nested virtualization, we have three levels: The host (KVM), which we call 33L0, the guest hypervisor, which we call L1, and its nested guest, which we 34call L2. 35 36 37Running nested VMX 38------------------ 39 40The nested VMX feature is enabled by default since Linux kernel v4.20. For 41older Linux kernel, it can be enabled by giving the "nested=1" option to the 42kvm-intel module. 43 44 45No modifications are required to user space (qemu). However, qemu's default 46emulated CPU type (qemu64) does not list the "VMX" CPU feature, so it must be 47explicitly enabled, by giving qemu one of the following options: 48 49 - cpu host (emulated CPU has all features of the real CPU) 50 51 - cpu qemu64,+vmx (add just the vmx feature to a named CPU type) 52 53 54ABIs 55---- 56 57Nested VMX aims to present a standard and (eventually) fully-functional VMX 58implementation for the a guest hypervisor to use. As such, the official 59specification of the ABI that it provides is Intel's VMX specification, 60namely volume 3B of their "Intel 64 and IA-32 Architectures Software 61Developer's Manual". Not all of VMX's features are currently fully supported, 62but the goal is to eventually support them all, starting with the VMX features 63which are used in practice by popular hypervisors (KVM and others). 64 65As a VMX implementation, nested VMX presents a VMCS structure to L1. 66As mandated by the spec, other than the two fields revision_id and abort, 67this structure is *opaque* to its user, who is not supposed to know or care 68about its internal structure. Rather, the structure is accessed through the 69VMREAD and VMWRITE instructions. 70Still, for debugging purposes, KVM developers might be interested to know the 71internals of this structure; This is struct vmcs12 from arch/x86/kvm/vmx.c. 72 73The name "vmcs12" refers to the VMCS that L1 builds for L2. In the code we 74also have "vmcs01", the VMCS that L0 built for L1, and "vmcs02" is the VMCS 75which L0 builds to actually run L2 - how this is done is explained in the 76aforementioned paper. 77 78For convenience, we repeat the content of struct vmcs12 here. If the internals 79of this structure changes, this can break live migration across KVM versions. 80VMCS12_REVISION (from vmx.c) should be changed if struct vmcs12 or its inner 81struct shadow_vmcs is ever changed. 82 83:: 84 85 typedef u64 natural_width; 86 struct __packed vmcs12 { 87 /* According to the Intel spec, a VMCS region must start with 88 * these two user-visible fields */ 89 u32 revision_id; 90 u32 abort; 91 92 u32 launch_state; /* set to 0 by VMCLEAR, to 1 by VMLAUNCH */ 93 u32 padding[7]; /* room for future expansion */ 94 95 u64 io_bitmap_a; 96 u64 io_bitmap_b; 97 u64 msr_bitmap; 98 u64 vm_exit_msr_store_addr; 99 u64 vm_exit_msr_load_addr; 100 u64 vm_entry_msr_load_addr; 101 u64 tsc_offset; 102 u64 virtual_apic_page_addr; 103 u64 apic_access_addr; 104 u64 ept_pointer; 105 u64 guest_physical_address; 106 u64 vmcs_link_pointer; 107 u64 guest_ia32_debugctl; 108 u64 guest_ia32_pat; 109 u64 guest_ia32_efer; 110 u64 guest_pdptr0; 111 u64 guest_pdptr1; 112 u64 guest_pdptr2; 113 u64 guest_pdptr3; 114 u64 host_ia32_pat; 115 u64 host_ia32_efer; 116 u64 padding64[8]; /* room for future expansion */ 117 natural_width cr0_guest_host_mask; 118 natural_width cr4_guest_host_mask; 119 natural_width cr0_read_shadow; 120 natural_width cr4_read_shadow; 121 natural_width dead_space[4]; /* Last remnants of cr3_target_value[0-3]. */ 122 natural_width exit_qualification; 123 natural_width guest_linear_address; 124 natural_width guest_cr0; 125 natural_width guest_cr3; 126 natural_width guest_cr4; 127 natural_width guest_es_base; 128 natural_width guest_cs_base; 129 natural_width guest_ss_base; 130 natural_width guest_ds_base; 131 natural_width guest_fs_base; 132 natural_width guest_gs_base; 133 natural_width guest_ldtr_base; 134 natural_width guest_tr_base; 135 natural_width guest_gdtr_base; 136 natural_width guest_idtr_base; 137 natural_width guest_dr7; 138 natural_width guest_rsp; 139 natural_width guest_rip; 140 natural_width guest_rflags; 141 natural_width guest_pending_dbg_exceptions; 142 natural_width guest_sysenter_esp; 143 natural_width guest_sysenter_eip; 144 natural_width host_cr0; 145 natural_width host_cr3; 146 natural_width host_cr4; 147 natural_width host_fs_base; 148 natural_width host_gs_base; 149 natural_width host_tr_base; 150 natural_width host_gdtr_base; 151 natural_width host_idtr_base; 152 natural_width host_ia32_sysenter_esp; 153 natural_width host_ia32_sysenter_eip; 154 natural_width host_rsp; 155 natural_width host_rip; 156 natural_width paddingl[8]; /* room for future expansion */ 157 u32 pin_based_vm_exec_control; 158 u32 cpu_based_vm_exec_control; 159 u32 exception_bitmap; 160 u32 page_fault_error_code_mask; 161 u32 page_fault_error_code_match; 162 u32 cr3_target_count; 163 u32 vm_exit_controls; 164 u32 vm_exit_msr_store_count; 165 u32 vm_exit_msr_load_count; 166 u32 vm_entry_controls; 167 u32 vm_entry_msr_load_count; 168 u32 vm_entry_intr_info_field; 169 u32 vm_entry_exception_error_code; 170 u32 vm_entry_instruction_len; 171 u32 tpr_threshold; 172 u32 secondary_vm_exec_control; 173 u32 vm_instruction_error; 174 u32 vm_exit_reason; 175 u32 vm_exit_intr_info; 176 u32 vm_exit_intr_error_code; 177 u32 idt_vectoring_info_field; 178 u32 idt_vectoring_error_code; 179 u32 vm_exit_instruction_len; 180 u32 vmx_instruction_info; 181 u32 guest_es_limit; 182 u32 guest_cs_limit; 183 u32 guest_ss_limit; 184 u32 guest_ds_limit; 185 u32 guest_fs_limit; 186 u32 guest_gs_limit; 187 u32 guest_ldtr_limit; 188 u32 guest_tr_limit; 189 u32 guest_gdtr_limit; 190 u32 guest_idtr_limit; 191 u32 guest_es_ar_bytes; 192 u32 guest_cs_ar_bytes; 193 u32 guest_ss_ar_bytes; 194 u32 guest_ds_ar_bytes; 195 u32 guest_fs_ar_bytes; 196 u32 guest_gs_ar_bytes; 197 u32 guest_ldtr_ar_bytes; 198 u32 guest_tr_ar_bytes; 199 u32 guest_interruptibility_info; 200 u32 guest_activity_state; 201 u32 guest_sysenter_cs; 202 u32 host_ia32_sysenter_cs; 203 u32 padding32[8]; /* room for future expansion */ 204 u16 virtual_processor_id; 205 u16 guest_es_selector; 206 u16 guest_cs_selector; 207 u16 guest_ss_selector; 208 u16 guest_ds_selector; 209 u16 guest_fs_selector; 210 u16 guest_gs_selector; 211 u16 guest_ldtr_selector; 212 u16 guest_tr_selector; 213 u16 host_es_selector; 214 u16 host_cs_selector; 215 u16 host_ss_selector; 216 u16 host_ds_selector; 217 u16 host_fs_selector; 218 u16 host_gs_selector; 219 u16 host_tr_selector; 220 }; 221 222 223Authors 224------- 225 226These patches were written by: 227 - Abel Gordon, abelg <at> il.ibm.com 228 - Nadav Har'El, nyh <at> il.ibm.com 229 - Orit Wasserman, oritw <at> il.ibm.com 230 - Ben-Ami Yassor, benami <at> il.ibm.com 231 - Muli Ben-Yehuda, muli <at> il.ibm.com 232 233With contributions by: 234 - Anthony Liguori, aliguori <at> us.ibm.com 235 - Mike Day, mdday <at> us.ibm.com 236 - Michael Factor, factor <at> il.ibm.com 237 - Zvi Dubitzky, dubi <at> il.ibm.com 238 239And valuable reviews by: 240 - Avi Kivity, avi <at> redhat.com 241 - Gleb Natapov, gleb <at> redhat.com 242 - Marcelo Tosatti, mtosatti <at> redhat.com 243 - Kevin Tian, kevin.tian <at> intel.com 244 - and others. 245