Re: [PATCH 17/24] x86/entry: Introduce struct ist_regs

[Lai Jiangshan]

On 2021/9/1 01:50, Lai Jiangshan wrote:
> From: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
>
> struct ist_regs is the upmost stack frame for IST interrupt, it
> consists of struct pt_regs and other fields which will be added in
> later patch.
>
> Make vc_switch_off_ist() take ist_regs as its argument and it will switch
> the whole ist_regs if needed.
>
> Make the frame before calling paranoid_entry() and paranoid_exit() be
> struct ist_regs.
>
> This patch is prepared for converting paranoid_entry() and paranoid_exit()
> into C code which will need the additinal fields to store the results in
> paranoid_entry() and to use them in paranoid_exit().

This patch was over designed.

In ASM code, we can easily save results in the callee-saved registers.
For example, rc3 is saved in %r14, gsbase info is saved in %rbx.

And in C code, we can't save results in registers.  And I thought there was
no place to save the results because the CR3 and gsbase are not kernel's.
So I extended the pt_regs to ist_regs to save the results.

But it was incorrect.  The results can be saved in percpu data at the end of
paranoid_entry() after the CR3/gsbase are settled down.  And the results
can be read at the beginning of paranoid_exit() before the CR3/gsbase are
switched to the interrupted context's.

sigh.

> The C interrupt handlers don't use struct ist_regs due to they don't need
> the additional fields in the struct ist_regs, and they still use pt_regs.