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

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Message-ID: <befeb81d-234b-4e96-9966-2c0b11a78c79@zytor.com>
Date: Tue, 5 Nov 2024 13:58:40 -0800
From: "H. Peter Anvin" <hpa@...or.com>
To: "Woodhouse, David" <dwmw@...zon.co.uk>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "kexec@...ts.infradead.org" <kexec@...ts.infradead.org>,
        "jpoimboe@...nel.org" <jpoimboe@...nel.org>
Cc: "horms@...nel.org" <horms@...nel.org>, "x86@...nel.org" <x86@...nel.org>,
        "bp@...en8.de" <bp@...en8.de>, "mingo@...hat.com" <mingo@...hat.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "kai.huang@...el.com" <kai.huang@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "nik.borisov@...e.com" <nik.borisov@...e.com>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>
Subject: Re: [RFC PATCH 6/7] x86/kexec: Debugging support: Dump registers on
 exception

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();
}