Date:   Sat, 09 Jul 2022 20:41:47 +0200
From:   "Fabio M. De Francesco" <fmdefrancesco@...il.com>
To:     linux-kernel@...r.kernel.org, x86@...nel.org,
        kexec@...ts.infradead.org, ebiederm@...ssion.com,
        dyoung@...hat.com, bhe@...hat.com, vgoyal@...hat.com,
        Eric DeVolder <eric.devolder@...cle.com>
Cc:     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, ira.weiny@...el.com
Subject: Re: [PATCH v9 7/7] x86/crash: Add x86 crash hotplug support

On venerdì 8 luglio 2022 23:07:20 CEST Eric DeVolder wrote:
> 
> On 7/8/22 14:38, Fabio M. De Francesco wrote:
> > On martedì 14 giugno 2022 00:42:40 CEST 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.
> >>
> >> When loading the crash kernel via kexec_load or kexec_file_load,
> >> the elfcorehdr is identified at run time in
> >> crash_core:handle_hotplug_event().
> >>
> >> 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 then installed over the top of
> >> the existing elfcorehdr.
> >>
> >> In the patch 'kexec: 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_MAX_MEMORY_RANGES configure item.
> >>
> >> With this change, crash hotplug for kexec_file_load syscall
> >> is supported. The kexec_load is also supported, but also
> >> requires a corresponding change to userspace kexec-tools.
> >>
> >> Signed-off-by: Eric DeVolder <eric.devolder@...cle.com>
> >> ---
> >>   arch/x86/Kconfig        |  11 ++++
> >>   arch/x86/kernel/crash.c | 116 ++++++++++++++++++++++++++++++++++++++
++
> >>   2 files changed, 127 insertions(+)
> >>
> >> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> >> index 762a0b6ab8b6..e9eecff3b97d 100644
> >> --- a/arch/x86/Kconfig
> >> +++ b/arch/x86/Kconfig
> >> @@ -2082,6 +2082,17 @@ config CRASH_DUMP
> >>   	  (CONFIG_RELOCATABLE=y).
> >>   	  For more details see Documentation/admin-guide/kdump/kdump.rst
> >>   
> >> +config CRASH_MAX_MEMORY_RANGES
> >> +	depends on CRASH_DUMP && KEXEC_FILE && (HOTPLUG_CPU ||
> > MEMORY_HOTPLUG)
> >> +	int
> >> +	default 32768
> >> +	help
> >> +	  For the kexec_file_load path, specify the maximum number of
> >> +	  memory regions, eg. as represented by the 'System RAM' entries
> >> +	  in /proc/iomem, that the elfcorehdr buffer/segment can
> > accommodate.
> >> +	  This value is combined with NR_CPUS and multiplied by
> > Elf64_Phdr
> >> +	  size to determine the final buffer size.
> >> +
> >>   config KEXEC_JUMP
> >>   	bool "kexec jump"
> >>   	depends on KEXEC && HIBERNATION
> >> diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c
> >> index 9db41cce8d97..b9cdf7a0d868 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;
> >>   
> >> +#if defined(CONFIG_HOTPLUG_CPU) || defined(CONFIG_MEMORY_HOTPLUG)
> >> +	/* Ensure elfcorehdr segment large enough for hotplug changes */
> >> +	kbuf.memsz = (CONFIG_NR_CPUS_DEFAULT +
> > CONFIG_CRASH_MAX_MEMORY_RANGES) * sizeof(Elf64_Phdr);
> >> +	/* For marking as usable to crash kernel */
> >> +	image->elf_headers_sz = kbuf.memsz;
> >> +	/* Record the index of the elfcorehdr segment */
> >> +	image->elfcorehdr_index = image->nr_segments;
> >> +	image->elfcorehdr_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 +424,108 @@ int crash_load_segments(struct kimage *image)
> >>   	return ret;
> >>   }
> >>   #endif /* CONFIG_KEXEC_FILE */
> >> +
> >> +#if defined(CONFIG_HOTPLUG_CPU) || defined(CONFIG_MEMORY_HOTPLUG)
> >> +int crash_hotplug_support(void) { return 1; }
> >> +void *arch_map_crash_pages(unsigned long paddr, unsigned long size)
> >> +{
> >> +	/*
> >> +	 * NOTE: The addresses and sizes passed to this routine have
> >> +	 * already been fully aligned on page boundaries. There is no
> >> +	 * need for massaging the address or size.
> >> +	 */
> >> +	void *ptr = NULL;
> >> +
> >> +	/* NOTE: requires arch_kexec_[un]protect_crashkres() for write
> > access */
> >> +	if (size > 0) {
> >> +		struct page *page = pfn_to_page(paddr >> PAGE_SHIFT);
> >> +
> >> +		ptr = kmap(page);
> >> +	}
> > 
> > kmap_local_page() is preferred over kmap() which should be avoided in 
new
> > code (please take a look at highmem.rst which I recently updated).
> > 
> > Can you please check if you can easily avoid using kmap()? Otherwise, 
this
> > code will be added to my (long) queue and changed one day or the other 
:-)
> > 
> > Thanks,
> > 
> > Fabio
> Fabio,
> I converted this to kmap_local_page()!
> Thanks!
> eric
> 
> > 
Eric,

Thanks for the very quick reaction to my suggestion.

Fabio

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