关于linux x86 64中MSR_GS_BASE的详细信息

发布时间：2020-12-14 00:28:29 所属栏目：Linux 来源：网络整理

导读：我试图弄清楚 Linux内核中MACRO当前的细节. 当前的最终汇编代码是： movq %%gs:0xb000,%0 上面的代码可以工作！但是当我打印%% gs时,它的值为0,所以%% gs指向GDT NULL的第一项!!这个怎么运作？ mov %%gs,%0 相反,gs的基础在MSR_GS_BASE中,并且当前可以替换为

我试图弄清楚 Linux内核中MACRO当前的细节.
当前的最终汇编代码是：

movq %%gs:0xb000,%0

上面的代码可以工作！但是当我打印%% gs时,它的值为0,所以%% gs指向GDT NULL的第一项!!这个怎么运作？

mov %%gs,%0

相反,gs的基础在MSR_GS_BASE中,并且当前可以替换为：

/*0xb000 is the offset of per_cpu__current_task*/
cur_task = (unsigned long*)(x86_rdmsr64(MSR_GS_BASE) + 0xb000);
println("cur_task:%p",*cur_task);

我的问题是：

％gs指向GDT NULL的第一项!! ??如何从MSR_GS_BASE读取它是一个CPU功能？我需要一些关于此的参考.

解决方法

从 AMD Architecture Programmer’s Manual Volume 2: System Programming开始,第4.5.3节：

FS and GS Registers in 64-Bit Mode. Unlike the CS,DS,ES,and SS segments,the FS and GS segment overrides can be used in 64-bit mode. When FS and GS segment overrides are used in 64-bit mode,their respective base addresses are used in the effective-address (EA) calculation. The complete EA calculation then becomes (FS or GS).base + base + (scale * index) + displacement. The FS.base and GS.base values are also expanded to the full 64-bit virtual-address size,as shown in Figure 4-5. The resulting EA calculation is allowed to wrap across positive and negative addresses.

[…]

There are two methods to update the contents of the FS.base and GS.base hidden descriptor fields. The first is available exclusively to privileged software (CPL = 0). The FS.base and GS.base hidden descriptor-register fields are mapped to MSRs. Privileged software can load a 64-bit base address in canonical form into FS.base or GS.base using a single WRMSR instruction. The FS.base MSR address is C000_0100h while the GS.base MSR address is C000_0101h.

The second method of updating the FS and GS base fields is available to software running at any privilege level (when supported by the implementation and enabled by setting CR4[FSGSBASE]). The WRFSBASE and WRGSBASE instructions copy the contents of a GPR to the FS.base and GS.base fields respectively. When the operand size is 32 bits,the upper doubleword of the base is cleared. WRFSBASE and WRGSBASE are only supported in 64-bit mode.

（编辑：李大同）

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