linux-kernel - Re: [PATCH 1/7] kexec_file: add kexec_file flag to control debug printing

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Message-ID: <1e7863ec4e4ab10b84fd0e64f30f8464d2e484a3.camel@perches.com>
Date:   Tue, 14 Nov 2023 08:03:17 -0800
From:   Joe Perches <joe@...ches.com>
To:     Baoquan He <bhe@...hat.com>, linux-kernel@...r.kernel.org
Cc:     kexec@...ts.infradead.org, x86@...nel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-riscv@...ts.infradead.org, linuxppc-dev@...ts.ozlabs.org,
        linux-parisc@...r.kernel.org
Subject: Re: [PATCH 1/7] kexec_file: add kexec_file flag to control debug
 printing

On Tue, 2023-11-14 at 23:32 +0800, Baoquan He wrote:
> When specifying 'kexec -c -d', kexec_load interface will print loading
> information, e.g the regions where kernel/initrd/purgatory/cmdline
> are put, the memmap passed to 2nd kernel taken as system RAM ranges,
> and printing all contents of struct kexec_segment, etc. These are
> very helpful for analyzing or positioning what's happening when
> kexec/kdump itself failed. The debugging printing for kexec_load
> interface is made in user space utility kexec-tools.
> 
> Whereas, with kexec_file_load interface, 'kexec -s -d' print nothing.
> Because kexec_file code is mostly implemented in kernel space, and the
> debugging printing functionality is missed. It's not convenient when
> debugging kexec/kdump loading and jumping with kexec_file_load
> interface.
> 
> Now add KEXEC_FILE_DEBUG to kexec_file flag to control the debugging
> message printing. And add global variable kexec_file_dbg_print and
> macro kexec_dprintk() to facilitate the printing.
> 
> This is a preparation, later kexec_dprintk() will be used to replace the
> existing pr_debug(). Once 'kexec -s -d' is specified, it will print out
> kexec/kdump loading information. If '-d' is not specified, it regresses
> to pr_debug().

Not quite as pr_debug is completely eliminated with
zero object size when DEBUG is not #defined.

Now the object size will be larger and contain the
formats in .text.

[]
> diff --git a/include/linux/kexec.h b/include/linux/kexec.h
[]
> @@ -264,6 +264,18 @@ arch_kexec_apply_relocations(struct purgatory_info *pi, Elf_Shdr *section,
>  	return -ENOEXEC;
>  }
>  #endif
> +
> +extern bool kexec_file_dbg_print;
> +
> +#define kexec_dprintk(fmt, args...)			\
> +	do {						\
> +		if (kexec_file_dbg_print)		\
> +			printk(KERN_INFO fmt, ##args);	\
> +		else					\
> +			printk(KERN_DEBUG fmt, ##args);	\
> +	} while (0)
> +
> +

I don't know how many of these printks exist and if
overall object size matters but using

#define kexec_dprintkfmt, ...)					\
	printk("%s" fmt,					\
	       kexec_file_dbg_print ? KERN_INFO : KERN_DEBUG,	\
	       ##__VA_ARGS__)

should reduce overall object size by eliminating the
mostly duplicated format in .text which differs only
by the KERN_<PREFIX>