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SubjectRE: Kernel stack

>You discussed that kernel do not keep track of SS for process specific
>kernel stack as it always starts from fixed offset of task_struct, but
>does that mean, linux kernel do not make use of SS element in the TSS of
>process. I think the kernel can not by-pass the rules defined by
>processor. Processor expects the SS and SP elements to be right at the
>time of stack switching, so we need to initialize them to the right
>values while forking a process.

Yes, the kernel cannot bypass the TSS entirely.

>I think kernel definitely keep tracks of SS and SP for all CPU levels
>including kernel mode also (ring 0), so that when we are switching form
>user space to kernel space the processor can switch the stacks
>automatically. At stack switching time CPU expects the SS and SP
>elements of TSS to be right, it's just going to copy those values in SS
>and SP registers of CPU, so that now we can push and pop the things on
>the kernel stack.
In Linxu , the SS and DS normally holds the same value. Mostly I
have seen that,\
when in kernel mode, DS=SS=0x18 and when in user mode DS=SS=0x28
(this is what i remember !).

As Linux does not believe in x86 processor recommended task switching (ie
storing/retrieving all the
necessary register values in a TSS) it uses the process's stack to hold the
required registers.
But, it cannot get away from the fact that, when a switch happens to kernel
the processor expects to find the SS and ESP values for that privilege
level in the current TSS
(the one which i pointed to be the TS register if i am correct!).

In Linux, we have only one TSS per processor, and when a process is
scheduled most
probably the kernel sets the TSS's SS0 (SS corresponding to ring 0) as 0x18
and ESP0
will be current process's task_struct + 8K (in 2.4). Again, I have not
really gone through
that piece of code in the kernel, so this is my assumption. Correct me if I
am wrong.

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