Re: [PATCH v15 7/7] x86/crash: add x86 crash hotplug support

[Eric DeVolder]

On 1/4/23 03:08, Baoquan He wrote:
> On 12/09/22 at 10:36am, Eric DeVolder wrote:
> ...... snip out
> > diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c
> > index 9ceb93c176a6..5186df48ce6c 100644
> > --- a/arch/x86/kernel/crash.c
> > +++ b/arch/x86/kernel/crash.c
> > @@ -42,6 +42,21 @@
> >   #include <asm/crash.h>
> >   #include <asm/cmdline.h>
> >   
> > +/*
> > + * For the kexec_file_load() syscall path, specify the maximum number of
> > + * memory regions that the elfcorehdr buffer/segment can accommodate.
> > + * These regions are obtained via walk_system_ram_res(); eg. the
> > + * 'System RAM' entries in /proc/iomem.
> > + * This value is combined with NR_CPUS_DEFAULT and multiplied by
> > + * sizeof(Elf64_Phdr) to determine the final elfcorehdr memory buffer/
> > + * segment size.
> > + * The value 8192, for example, covers a (sparsely populated) 1TiB system
> > + * consisting of 128MiB memblocks, while resulting in an elfcorehdr
> > + * memory buffer/segment size under 1MiB. This represents a sane choice
> > + * to accommodate both baremetal and virtual machine configurations.
> > + */
> > +#define CRASH_MAX_MEMORY_RANGES 8192
> > +
> >   /* Used while preparing memory map entries for second kernel */
> >   struct crash_memmap_data {
> >   	struct boot_params *params;
> > @@ -394,10 +409,39 @@ 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;
> > +
> > +#ifdef CONFIG_CRASH_HOTPLUG
> > +	if (IS_ENABLED(CONFIG_HOTPLUG_CPU)) {
>
> Do I miss anything important during reviewing? I can't see why memory
> hotplug is also relying on HOTPLUG_CPU.

No, looks like I made a mistake. The ifdef needs to go, and the IS_ENABLED() immediately following 
needs to be CRASH_HOTPLUG. Having not looked at this in a few weeks, it's obvious now.
Thanks!
eric


> > +		/*
> > +		 * 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;
> > +
> > +		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);
> > +		}
> > +	}
> > +#endif
> >   
> > -	kbuf.memsz = kbuf.bufsz;
> >   	kbuf.buf_align = ELF_CORE_HEADER_ALIGN;
> >   	kbuf.mem = KEXEC_BUF_MEM_UNKNOWN;
> >   	ret = kexec_add_buffer(&kbuf);
> > @@ -412,3 +456,67 @@ int crash_load_segments(struct kimage *image)
> >   	return ret;
> >   }
> >   #endif /* CONFIG_KEXEC_FILE */
> > +
> > +#ifdef CONFIG_CRASH_HOTPLUG
> > +
> > +#undef pr_fmt
> > +#define pr_fmt(fmt) "crash hp: " fmt
> > +
> > +/**
> > + * arch_crash_handle_hotplug_event() - Handle hotplug elfcorehdr changes
> > + * @image: the active struct kimage
> > + *
> > + * To accurately reflect hot un/plug changes, the new elfcorehdr
> > + * is prepared in a kernel buffer, and then it is written on top
> > + * of the existing/old elfcorehdr.
> > + */
> > +void arch_crash_handle_hotplug_event(struct kimage *image)
> > +{
> > +	void *elfbuf = NULL, *old_elfcorehdr;
> > +	unsigned long mem, memsz;
> > +	unsigned long elfsz = 0;
> > +
> > +	/*
> > +	 * Create the new elfcorehdr reflecting the changes to CPU and/or
> > +	 * memory resources.
> > +	 */
> > +	if (prepare_elf_headers(image, &elfbuf, &elfsz)) {
> > +		pr_err("unable to prepare elfcore headers");
> > +		goto out;
> > +	}
> > +
> > +	/*
> > +	 * Obtain address and size of the elfcorehdr segment, and
> > +	 * check it against the new elfcorehdr buffer.
> > +	 */
> > +	mem = image->segment[image->elfcorehdr_index].mem;
> > +	memsz = image->segment[image->elfcorehdr_index].memsz;
> > +	if (elfsz > memsz) {
> > +		pr_err("update elfcorehdr elfsz %lu > memsz %lu",
> > +			elfsz, memsz);
> > +		goto out;
> > +	}
> > +
> > +	/*
> > +	 * Copy new elfcorehdr over the old elfcorehdr at destination.
> > +	 */
> > +	old_elfcorehdr = kmap_local_page(pfn_to_page(mem >> PAGE_SHIFT));
> > +	if (old_elfcorehdr) {
> > +		pr_err("updating elfcorehdr failed\n");
> > +		goto out;
> > +	}
> > +
> > +	/*
> > +	 * Temporarily invalidate the crash image while the
> > +	 * elfcorehdr is updated.
> > +	 */
> > +	xchg(&kexec_crash_image, NULL);
> > +	memcpy_flushcache(old_elfcorehdr, elfbuf, elfsz);
> > +	xchg(&kexec_crash_image, image);
> > +	kunmap_local(old_elfcorehdr);
> > +	pr_debug("updated elfcorehdr\n");
> > +
> > +out:
> > +	vfree(elfbuf);
> > +}
> > +#endif
> > --
> > 2.31.1