[M] Change in coreboot[main]: libpayload/x86: Extend exception handling to x86_64 architecture

[Subrata Banik (Code Review)]

Attention is currently required from: Julius Werner, Paul Menzel.

Subrata Banik has posted comments on this change by Subrata Banik. ( https://review.coreboot.org/c/coreboot/+/83036?usp=email )

Change subject: libpayload/x86: Extend exception handling to x86_64 architecture
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Patch Set 11:

(1 comment)

Patchset:

PS8:
> > Have you actually tried this out with fwgdb?
>
> [Subrata] Yes, i have used FW GDB to verify it works during exception.
> > I think that would be a good test, use GDB to manually change the values of RAX, RCX, RSP, etc. then step through one more instruction and then compare whether the registers still contain those new values you had it write. I think that would show most of the bugs you still had in here now (e.g. RCX not being restored from the struct correctly).
>
> [Subrata] I saw RCX still holds the old value correctly while existing from exception handler (irtq) although I missed to use `pop old_rcx` over `pop %rcx`. I believe that is due to restoring `%rcx` with old value (`old_rcs`) prior existing from exception.

Before calling into `exception_dispatch` function, the state of critical registers:

```
REG_IP:    0x0000000030023ef9
REG_FLAGS: 0x0000000000000046
REG_SP:    0x0000000034072ed8
REG_CX:    0x0000000003020100
CS:     0x0000000000000020
SS:     0x0000000000000018
```

Inside `exception_dispatch` function:

```
Debug Exception
Error code: n/a
REG_IP:    0x0000000030023ef9
REG_FLAGS: 0x0000000000000046
REG_AX:    0x0000000000000009
REG_BX:    0x0000000000000000
REG_CX:    0x0000000003020100
REG_DX:    0x0000000000000001
REG_SP:    0x0000000034072ed8
REG_BP:    0x0000000000000009
REG_SI:    0x0000000000000029
REG_DI:    0x0000000034072eef
REG_R8:    0x0000000000000009
REG_R9:    0x0000000000000000
REG_R10:   0x0000000000000000
REG_R11:   0x0000000034072d70
REG_R12:   0x0000000000000004
REG_R13:   0x0000000000000001
REG_R14:   0x0000000034072ee6
REG_R15:   0x0000000000000004
CS:     0x0020
DS:     0x0000
ES:     0x0000
SS:     0x0018
FS:     0x0018
GS:     0x0050
```

After existing from `exception_dispatch` function, the state of critical registers:
```
REG_IP:    0x0000000030023ed9
REG_FLAGS: 0x0000000000000046
REG_SP:    0x0000000034072ed8
REG_CX:    0x0000000003020100
CS:     0x0000000000000020
SS:     0x0000000000000018
```

I have ensured that the register values are unified before and after the exception_dispatch function  hence, marking this open resolved.