[RFC PATCH 6/7] x86/kexec: Debugging support: Dump registers on exception

[H. Peter Anvin]

On 11/5/24 13:37, H. Peter Anvin wrote:
> What is the point of writing this code in assembly in the first place? A 
> much more logical thing to do is to just push the registers you haven't 
> pushed already onto the stack and call a C function to do the actual 
> dumping? It isn't like it is in any shape, way or form performance 
> critical.

arch/x86/boot/compressed/misc.c has some code that you can crib, both 
for writing to a text screen (not that useful anymore with EFI 
framebuffers) and serial port. If you factor it a little bit then you 
can probably even share the code directly.

(__putstr perhaps should have a __putchar() factored out of it?)

Then you can just do the obvious (have your assembly stub point %rdi to 
the base of the register dump; set the frame order to whatever you'd 
like, except rip/err/exc, or reverse the order if you prefer by changing 
the loop):

static inline __noreturn void die(void)
{
	while (1)
		asm volatile("hlt");
}

void dump_register_frame(const unsigned long frame[])
{
	static const char regnames[][5] = {
		"rax:", "rcx:", "rdx:", "rbx:",
		"rsp:", "rbp:", "rsi:", "rdi:",
		"r8: ", "r9: ", "r10:", "r11:",
		"r12:", "r13:", "r14:", "r15:",
		"cr2:", "Exc:", "Err:", "rip:"
	};

	for (size_t i = 0; i < ARRAY_SIZE(regnames); i++) {
		__putstr(regnames[i]);
		__puthex(frame[i]);
		__putstr("\n");
	}

	/* Only return from int3 */
	if (frame[17] != 3)
		die();
}