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[求助] 请教kvm虚拟机, 在guest用户态读写msr寄存器时， 为什么没有触发vm exit？
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2021-4-27 16:15
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我认为一般是先检查异常,前置检查跑完了再给VMCS的 不相信的你可以试验一下,捕获PG异常 看看 |
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我测试了dr寄存器相关的mov指令: asm("movq %0,%%dr0"::"r"(0xaabbccddaa));
MOV — Move to/from Debug Registers MOV—Move to/from Debug Registers Moves the contents of a debug register (DR0, DR1, DR2, DR3, DR4, DR5, DR6, or DR7) to a general-purpose register or vice versa. The operand size for these instructions is always 32 bits in non-64-bit modes, regardless of the operand-size attribute. (See Section 17.2, “Debug Registers”, of the Intel® 64 and IA-32 Architectures Soft-ware Developer’s Manual, Volume 3A, for a detailed description of the flags and fields in the debug registers.) The instructions must be executed at privilege level 0 or in real-address mode. When the debug extension (DE) flag in register CR4 is clear, these instructions operate on debug registers in a manner that is compatible with Intel386 and Intel486 processors. In this mode, references to DR4 and DR5 refer to DR6 and DR7, respectively. When the DE flag in CR4 is set, attempts to reference DR4 and DR5 result in an undefined opcode (#UD) exception. (The CR4 register was added to the IA-32 Architecture beginning with the Pentium processor.) At the opcode level, the reg field within the ModR/M byte specifies which of the debug registers is loaded or read. The two bits in the mod field are ignored. The r/m field specifies the general-purpose register loaded or read. In 64-bit mode, the instruction’s default operation size is 64 bits. Use of the REX.B prefix permits access to addi-tional registers (R8–R15). Use of the REX.W or 66H prefix is ignored. Use of the REX.R prefix causes an invalid-opcode exception. See the summary chart at the beginning of this section for encoding data and limits. 64-Bit Mode Exceptions #GP(0) If the current privilege level is not 0. If an attempt is made to write a 1 to any of bits 63:32 in DR6. If an attempt is made to write a 1 to any of bits 63:32 in DR7 如果GP异常的优先级高于vm exit, 此处应该不会触发vm exit. 但是我在guest用户态确实触发了vm exit. static int handle_dr(struct kvm_vcpu *vcpu)
{
unsigned long exit_qualification;
int dr, reg;
/* Do not handle if the CPL > 0, will trigger GP on re-entry */
if (!kvm_require_cpl(vcpu, 0))// 此处检查了调用方是否来自ring0
return 1;
...
最后于 2021-4-28 10:53
被音货得福编辑
，原因：
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可是这个说明不是说必须是privilege level 0吗? |
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那个白帽网站是你开的不，进去是有个鸭子走来走去的那个 |
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我找到 《处理器虚拟化技术》看了, 确实存在这种说明, 但没在 《Intel® 64 and IA-32 ArchitecturesSoftware Developer’s Manual》找到类似说明，不知道这个说法有没有出处。另外我还关注到exception bitmap好像蛮重要，它也决定了一些情况下是否需要触发vm exit。
最后于 2021-4-28 18:07
被音货得福编辑
，原因：
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也感谢大佬的解答 |
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哈哈, 终于找到原文了, 这里明确说了用户态读写msr寄存器不会产生vm exit, 而 dr就会产生. 我也是真的刚好碰上到两个特例 -_-"" , 把我给绕晕了. |
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ookkaa 
