Re: [RFC PATCH 1/1] x86/traps: Get rid of exception handlers' second argument error code

[Peter Zijlstra]

On Fri, Aug 04, 2023 at 12:57:34AM -0700, Xin Li wrote:
> The IDT event delivery of X86_TRAP_DF, X86_TRAP_TS, X86_TRAP_NP,
> X86_TRAP_SS, X86_TRAP_GP, X86_TRAP_AC and X86_TRAP_CP pushes an error
> code into the orig_ax member of the pt_regs structure, and the error
> code is passed as the second argument of their C-handlers, although
> the pt_regs structure is already passed as the first argument.
> 
> The asm entry code of such faults does the following
> 
>   movq ORIG_RAX(%rsp), %rsi	/* get error code into 2nd argument*/
>   movq $-1, ORIG_RAX(%rsp)	/* no syscall to restart */
> 
> to set the orig_ax member to -1 just before calling the C-handler.
> 
> In addition, the IRQ entry code uses the second error code argument
> as its IRQ vector, as the IRQ asm entry code pushes its IRQ vector
> into the orig_ax member.
> 
> The commit d99015b1abbad ("x86: move entry_64.S register saving out of
> the macros") introduced the changes to set orig_ax to -1, but I can't
> see why it's required. Our tests on x86_64 and x86_32 seem fine if
> orig_ax is left unchanged instead of set to -1.

That means that SYSCALL_NUM(regs) get to be garbage; or something like
that.