AMD SVM Virtualization Explained: AMD's VT Solution
AMD SVM (Secure Virtual Machine) is AMD's answer to Intel VT-x. Centered on the VMCB data structure and the #VMEXIT event mechanism, it differs noticeably from Intel VMX in design and API.
Core Concepts
SVM Modes
SVM introduces host mode (hypervisor) and guest mode (guest), switched via VMRUN.
VMCB (Virtual Machine Control Block)
A 4KB memory region holding guest state, control area and intercept bitmap — the heart of SVM state.
The VMRUN Instruction
VMRUN takes the VMCB physical address as operand, loads guest state and starts guest execution.
#VMEXIT Events
When the guest triggers an intercepted event (CPUID, INVLPG, CR access, etc.), a #VMEXIT occurs; the CPU saves guest state into the VMCB and returns to host.
CLGI / STGI
STGI sets the global interrupt flag and CLGI clears it; CLGI typically guards host critical sections before VMRUN.
ASID (Address Space Identifier)
Like Intel VPID, it tags TLB entries per guest to avoid full TLB flushes after #VMEXIT.
Key Code
; AMD SVM 进入 guest 的最小流程（汇编示意）
mov eax, cr4
bts eax, 13 ; CR4.SVME = 1
mov cr4, eax
mov eax, cr0
bts eax, 12 ; CR0.SVME = 1
mov cr0, eax
clgi ; 关闭全局中断
; 填充 VMCB 的 guest 状态与控制区 ...
mov rax, vmcb_pa ; VMCB 物理地址
vmrun rax ; 进入 guest（#VMEXIT 后回到下一条）
; 读取 VMCB 的 EXITCODE 字段分发处理
stgi ; 恢复全局中断Structure Cheat Sheet
| VMRUN | Enters guest mode with a VMCB |
|---|---|
| #VMEXIT | Exit caused by intercepted events; state auto-saved to VMCB |
| VMMCALL | Guest-initiated call into host (hypercall-like) |
| CLGI / STGI | Clear / set the global interrupt flag |
| INVLPGA | Invalidates TLB entries by ASID |
| VMLOAD / VMSAVE | Selectively save/restore guest hidden state (FS/GS/KernelGSbase, etc.) |
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