linux-kernel - Re: [PATCH v16 7/7] x86/crash: add x86 crash hotplug support

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Date:   Mon, 9 Jan 2023 15:36:09 +0800
From:   Baoquan He <bhe@...hat.com>
To:     Eric DeVolder <eric.devolder@...cle.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        kexec@...ts.infradead.org, ebiederm@...ssion.com,
        dyoung@...hat.com, vgoyal@...hat.com, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        hpa@...or.com, nramas@...ux.microsoft.com, thomas.lendacky@....com,
        robh@...nel.org, efault@....de, rppt@...nel.org, david@...hat.com,
        sourabhjain@...ux.ibm.com, konrad.wilk@...cle.com,
        boris.ostrovsky@...cle.com
Subject: Re: [PATCH v16 7/7] x86/crash: add x86 crash hotplug support

On 01/05/23 at 10:17am, Eric DeVolder wrote:
......
> @@ -394,10 +409,37 @@ int crash_load_segments(struct kimage *image)
>  	if (ret)
>  		return ret;
>  
> -	image->elf_headers = kbuf.buffer;
> -	image->elf_headers_sz = kbuf.bufsz;
> +	image->elf_headers	= kbuf.buffer;
> +	image->elf_headers_sz	= kbuf.bufsz;
> +	kbuf.memsz		= kbuf.bufsz;
> +
> +	if (IS_ENABLED(CONFIG_CRASH_HOTPLUG)) {
> +		/*
> +		 * Ensure the elfcorehdr segment large enough for hotplug changes.
> +		 * Start with VMCOREINFO and kernel_map.
> +		 */
> +		unsigned long pnum = 2;
> +
> +		if (IS_ENABLED(CONFIG_HOTPLUG_CPU))
> +			pnum += CONFIG_NR_CPUS_DEFAULT;
> +
> +		if (IS_ENABLED(CONFIG_MEMORY_HOTPLUG))
> +			pnum += CRASH_MAX_MEMORY_RANGES;

Logic of pnum calculating is a little confusing to me. If I only enable
one of CONFIG_HOTPLUG_CPU and CONFIG_MEMORY_HOTPLUG, is it OK? Say I
only enable CONFIG_HOTPLUG_CPU and CONFIG_SMP, on x86_64,
CONFIG_NR_CPUS_DEFAULT will be 64. pnum will be 64, is it OK. Am I miss
anything?

> +
> +		if (pnum < (unsigned long)PN_XNUM) {
> +			kbuf.memsz = pnum * sizeof(Elf64_Phdr);
> +			kbuf.memsz += sizeof(Elf64_Ehdr);
> +
> +			image->elfcorehdr_index = image->nr_segments;
> +			image->elfcorehdr_index_valid = true;
> +
> +			/* Mark as usable to crash kernel, else crash kernel fails on boot */
> +			image->elf_headers_sz = kbuf.memsz;
> +		} else {
> +			pr_err("number of Phdrs %lu exceeds max\n", pnum);
> +		}
> +	}
>  
> -	kbuf.memsz = kbuf.bufsz;
>  	kbuf.buf_align = ELF_CORE_HEADER_ALIGN;
>  	kbuf.mem = KEXEC_BUF_MEM_UNKNOWN;
>  	ret = kexec_add_buffer(&kbuf);
> @@ -412,3 +454,67 @@ int crash_load_segments(struct kimage *image)
>  	return ret;
>  }
>  #endif /* CONFIG_KEXEC_FILE */