Re: [PATCH v4 09/10] crash hp: Add x86 crash hotplug support for kexec_file_load

[Baoquan He]

On 03/01/22 at 02:12pm, Eric DeVolder wrote:
> 
> 
> On 2/22/22 22:10, Baoquan He wrote:
> > On 02/09/22 at 02:57pm, Eric DeVolder wrote:
> > > For x86_64, when CPU or memory is hot un/plugged, the crash
> > > elfcorehdr, which describes the CPUs and memory in the system,
> > > must also be updated.
> > 
> > Please mark the patch as X86 related by adding:
> > 
> > x86/kexec: Add x86 crash hotplug support for kexec_file_load
> Done!
> > 
> > And also update other subject to reflect under which component the
> > changes are made, crash hp is the feature name.
> Here's what I have at the moment, is this matching expectation?
> 
> pick 891fcff41a8f crash: fix minor typo/bug in debug message

It's x86 code change, shoud be:
  x86/crash: fix minor typo/bug in debug message

Others are similar.

> pick 8f44e0d695be crash hp: Introduce CRASH_HOTPLUG configuration options
> pick bae60e9db6ad crash: prototype change for crash_prepare_elf64_headers
> pick cb98d1d923c8 crash: generic crash hotplug support infrastructure
> pick 40e4434abad4 kexec: exclude elfcorehdr from the segment digest
> pick 75039922aedc kexec: exclude hot remove cpu from elfcorehdr notes
> pick 0be483dadeb8 x86/kexec: Add x86 crash hotplug support for kexec_file_load
> pick 388bb5e4bf9a x86/kexec: Add x86 crash hotplug support for kexec_load
> 
> > 
> > Otherwise, this looks good to me.
> Thanks!
> eric
> 
> > 
> > > 
> > > To update the elfcorehdr for x86_64, a new elfcorehdr must be
> > > generated from the available CPUs and memory. The new elfcorehdr
> > > is prepared into a buffer, and if no errors occur, it is
> > > installed over the top of the existing elfcorehdr.
> > > 
> > > In the patch 'crash hp: exclude elfcorehdr from the segment digest'
> > > the need to update purgatory due to the change in elfcorehdr was
> > > eliminated.  As a result, no changes to purgatory or boot_params
> > > (as the elfcorehdr= kernel command line parameter pointer
> > > remains unchanged and correct) are needed, just elfcorehdr.
> > > 
> > > To accommodate a growing number of resources via hotplug, the
> > > elfcorehdr segment must be sufficiently large enough to accommodate
> > > changes, see the CRASH_HOTPLUG_ELFCOREHDR_SZ configure item.
> > > 
> > > With this change, the kexec_file_load syscall (not kexec_load)
> > > is supported. When loading the crash kernel via kexec_file_load,
> > > the elfcorehdr is identified at load time in crash_load_segments().
> > > 
> > > Signed-off-by: Eric DeVolder <eric.devolder@xxxxxxxxxx>
> > > ---
> > >   arch/x86/kernel/crash.c | 92 +++++++++++++++++++++++++++++++++++++++++
> > >   1 file changed, 92 insertions(+)
> > > 
> > > diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c
> > > index 9db41cce8d97..6215c51919db 100644
> > > --- a/arch/x86/kernel/crash.c
> > > +++ b/arch/x86/kernel/crash.c
> > > @@ -25,6 +25,7 @@
> > >   #include <linux/slab.h>
> > >   #include <linux/vmalloc.h>
> > >   #include <linux/memblock.h>
> > > +#include <linux/highmem.h>
> > >   #include <asm/processor.h>
> > >   #include <asm/hardirq.h>
> > > @@ -398,7 +399,17 @@ int crash_load_segments(struct kimage *image)
> > >   	image->elf_headers = kbuf.buffer;
> > >   	image->elf_headers_sz = kbuf.bufsz;
> > > +#ifdef CONFIG_CRASH_HOTPLUG
> > > +	/* Ensure elfcorehdr segment large enough for hotplug changes */
> > > +	kbuf.memsz = CONFIG_CRASH_HOTPLUG_ELFCOREHDR_SZ;
> > > +	/* For marking as usable to crash kernel */
> > > +	image->elf_headers_sz = kbuf.memsz;
> > > +	/* Record the index of the elfcorehdr segment */
> > > +	image->elf_index = image->nr_segments;
> > > +	image->elf_index_valid = true;
> > > +#else
> > >   	kbuf.memsz = kbuf.bufsz;
> > > +#endif
> > >   	kbuf.buf_align = ELF_CORE_HEADER_ALIGN;
> > >   	kbuf.mem = KEXEC_BUF_MEM_UNKNOWN;
> > >   	ret = kexec_add_buffer(&kbuf);
> > > @@ -413,3 +424,84 @@ int crash_load_segments(struct kimage *image)
> > >   	return ret;
> > >   }
> > >   #endif /* CONFIG_KEXEC_FILE */
> > > +
> > > +#ifdef CONFIG_CRASH_HOTPLUG
> > > +/**
> > > + * arch_crash_hotplug_handler() - Handle hotplug elfcorehdr changes
> > > + * @image: the active struct kimage
> > > + * @hp_action: the hot un/plug action being handled
> > > + * @a: first parameter dependent upon hp_action
> > > + * @b: first parameter dependent upon hp_action
> > > + *
> > > + * To accurately reflect hot un/plug changes, the elfcorehdr (which
> > > + * is passed to the crash kernel via the elfcorehdr= parameter)
> > > + * must be updated with the new list of CPUs and memories. The new
> > > + * elfcorehdr is prepared in a kernel buffer, and if no errors,
> > > + * then it is written on top of the existing/old elfcorehdr.
> > > + *
> > > + * For hotplug changes to elfcorehdr to work, two conditions are
> > > + * needed:
> > > + * First, the segment containing the elfcorehdr must be large enough
> > > + * to permit a growing number of resources. See
> > > + * CONFIG_CRASH_HOTPLUG_ELFCOREHDR_SZ.
> > > + * Second, purgatory must explicitly exclude the elfcorehdr from the
> > > + * list of segments it checks (since the elfcorehdr changes and thus
> > > + * would require an update to purgatory itself to update the digest).
> > > + *
> > > + */
> > > +void arch_crash_hotplug_handler(struct kimage *image,
> > > +	unsigned int hp_action, unsigned long a, unsigned long b)
> > > +{
> > > +	struct kexec_segment *ksegment;
> > > +	unsigned char *ptr = NULL;
> > > +	unsigned long elfsz = 0;
> > > +	void *elfbuf = NULL;
> > > +	unsigned long mem, memsz;
> > > +
> > > +	/* Must have valid elfcorehdr index */
> > > +	if (!image->elf_index_valid) {
> > > +		pr_err("crash hp: unable to locate elfcorehdr segment");
> > > +		goto out;
> > > +	}
> > > +
> > > +	ksegment = &image->segment[image->elf_index];
> > > +	mem = ksegment->mem;
> > > +	memsz = ksegment->memsz;
> > > +
> > > +	/*
> > > +	 * Create the new elfcorehdr reflecting the changes to CPU and/or
> > > +	 * memory resources. The elfcorehdr segment memsz must be
> > > +	 * sufficiently large to accommodate increases due to hotplug
> > > +	 * activity. See CRASH_HOTPLUG_ELFCOREHDR_SZ.
> > > +	 */
> > > +	if (prepare_elf_headers(image, &elfbuf, &elfsz)) {
> > > +		pr_err("crash hp: unable to prepare elfcore headers");
> > > +		goto out;
> > > +	}
> > > +	if (elfsz > memsz) {
> > > +		pr_err("crash hp: update elfcorehdr elfsz %lu > memsz %lu",
> > > +			elfsz, memsz);
> > > +		goto out;
> > > +	}
> > > +
> > > +	/*
> > > +	 * At this point, we are all but assured of success.
> > > +	 * Copy new elfcorehdr into destination.
> > > +	 */
> > > +	ptr = map_crash_pages(mem, memsz);
> > > +	if (ptr) {
> > > +		/* Temporarily invalidate the crash image while it is replaced */
> > > +		xchg(&kexec_crash_image, NULL);
> > > +		/* Write the new elfcorehdr into memory */
> > > +		memcpy_flushcache((void *)ptr, elfbuf, elfsz);
> > > +		/* The crash image is now valid once again */
> > > +		xchg(&kexec_crash_image, image);
> > > +	}
> > > +	unmap_crash_pages((void **)&ptr);
> > > +	pr_debug("crash hp: re-loaded elfcorehdr at 0x%lx\n", mem);
> > > +
> > > +out:
> > > +	if (elfbuf)
> > > +		vfree(elfbuf);
> > > +}
> > > +#endif /* CONFIG_CRASH_HOTPLUG */
> > > -- 
> > > 2.27.0
> > > 
> > 
>