关于linux x86 64中MSR_GS_BASE的详细信息
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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.
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