Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
From:   "Fabio M. De Francesco" <fmdefrancesco@gmail.com>
To:     linux-kernel@vger.kernel.org, x86@kernel.org,
        kexec@lists.infradead.org, ebiederm@xmission.com,
        dyoung@redhat.com, bhe@redhat.com, vgoyal@redhat.com,
        Eric DeVolder <eric.devolder@oracle.com>
Cc:     tglx@linutronix.de, mingo@redhat.com, bp@alien8.de,
        dave.hansen@linux.intel.com, hpa@zytor.com,
        nramas@linux.microsoft.com, thomas.lendacky@amd.com,
        robh@kernel.org, efault@gmx.de, rppt@kernel.org, david@redhat.com,
        sourabhjain@linux.ibm.com, konrad.wilk@oracle.com,
        boris.ostrovsky@oracle.com, eric.devolder@oracle.com
Subject: Re: [PATCH v9 7/7] x86/crash: Add x86 crash hotplug support
Date:   Fri, 08 Jul 2022 21:38:27 +0200
Message-ID: <2843978.e9J7NaK4W3@opensuse>
In-Reply-To: <20220613224240.79400-8-eric.devolder@oracle.com>
References: <20220613224240.79400-1-eric.devolder@oracle.com> <20220613224240.79400-8-eric.devolder@oracle.com>
MIME-Version: 1.0
Content-Transfer-Encoding: quoted-printable
Content-Type: text/plain; charset="utf-8"
Precedence: bulk

On marted=C3=AC 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.
>=20
> 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().
>=20
> 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.
>=20
> 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=3D kernel command line parameter pointer
> remains unchanged and correct) are needed, just elfcorehdr.
>=20
> 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.
>=20
> 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.
>=20
> Signed-off-by: Eric DeVolder <eric.devolder@oracle.com>
> ---
>  arch/x86/Kconfig        |  11 ++++
>  arch/x86/kernel/crash.c | 116 ++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 127 insertions(+)
>=20
> 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=3Dy).
>  	  For more details see Documentation/admin-guide/kdump/kdump.rst
> =20
> +config CRASH_MAX_MEMORY_RANGES
> +	depends on CRASH_DUMP && KEXEC_FILE && (HOTPLUG_CPU ||=20
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=20
accommodate.
> +	  This value is combined with NR_CPUS and multiplied by=20
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>
> =20
>  #include <asm/processor.h>
>  #include <asm/hardirq.h>
> @@ -398,7 +399,17 @@ int crash_load_segments(struct kimage *image)
>  	image->elf_headers =3D kbuf.buffer;
>  	image->elf_headers_sz =3D kbuf.bufsz;
> =20
> +#if defined(CONFIG_HOTPLUG_CPU) || defined(CONFIG_MEMORY_HOTPLUG)
> +	/* Ensure elfcorehdr segment large enough for hotplug changes */
> +	kbuf.memsz =3D (CONFIG_NR_CPUS_DEFAULT +=20
CONFIG_CRASH_MAX_MEMORY_RANGES) * sizeof(Elf64_Phdr);
> +	/* For marking as usable to crash kernel */
> +	image->elf_headers_sz =3D kbuf.memsz;
> +	/* Record the index of the elfcorehdr segment */
> +	image->elfcorehdr_index =3D image->nr_segments;
> +	image->elfcorehdr_index_valid =3D true;
> +#else
>  	kbuf.memsz =3D kbuf.bufsz;
> +#endif
>  	kbuf.buf_align =3D ELF_CORE_HEADER_ALIGN;
>  	kbuf.mem =3D KEXEC_BUF_MEM_UNKNOWN;
>  	ret =3D 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 =3D NULL;
> +
> +	/* NOTE: requires arch_kexec_[un]protect_crashkres() for write=20
access */
> +	if (size > 0) {
> +		struct page *page =3D pfn_to_page(paddr >> PAGE_SHIFT);
> +
> +		ptr =3D kmap(page);
> +	}

kmap_local_page() is preferred over kmap() which should be avoided in new=20
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=
=20
code will be added to my (long) queue and changed one day or the other :-)

Thanks,

=46abio

> +
> +	return ptr;
> +}
> +
> +void arch_unmap_crash_pages(void **ptr)
> +{
> +	if (ptr) {
> +		if (*ptr)
> +			kunmap(*ptr);
> +		*ptr =3D NULL;
> +	}
> +}
> +
> +/**
> + * arch_crash_handle_hotplug_event() - Handle hotplug elfcorehdr changes
> + * @image: the active struct kimage
> + * @hp_action: the hot un/plug action being handled
> + * @cpu: when KEXEC_CRASH_HP_ADD/REMOVE_CPU, the cpu affected
> + *
> + * To accurately reflect hot un/plug changes, the elfcorehdr (which
> + * is passed to the crash kernel via the elfcorehdr=3D parameter)
> + * must be updated with the new list of CPUs and memories. The new
> + * elfcorehdr is prepared in a kernel buffer, and 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 the
> + * CONFIG_CRASH_MAX_MEMORY_RANGES description.
> + * 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_handle_hotplug_event(struct kimage *image,
> +	unsigned int hp_action, unsigned int cpu)
> +{
> +	struct kexec_segment *ksegment;
> +	unsigned char *ptr =3D NULL;
> +	unsigned long elfsz =3D 0;
> +	void *elfbuf =3D NULL;
> +	unsigned long mem, memsz;
> +
> +	/*
> +	 * Elfcorehdr_index_valid checked in=20
crash_core:handle_hotplug_event()
> +	 */
> +	ksegment =3D &image->segment[image->elfcorehdr_index];
> +	mem =3D ksegment->mem;
> +	memsz =3D ksegment->memsz;
> +
> +	/*
> +	 * Create the new elfcorehdr reflecting the changes to CPU and/or
> +	 * memory resources.
> +	 */
> +	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=20
%lu",
> +			elfsz, memsz);
> +		goto out;
> +	}
> +
> +	/*
> +	 * At this point, we are all but assured of success.
> +	 * Copy new elfcorehdr into destination.
> +	 */
> +	ptr =3D arch_map_crash_pages(mem, memsz);
> +	if (ptr) {
> +		/*
> +		 * Temporarily invalidate the crash image while the
> +		 * elfcorehdr is updated.
> +		 */
> +		xchg(&kexec_crash_image, NULL);
> +		memcpy_flushcache((void *)ptr, elfbuf, elfsz);
> +		xchg(&kexec_crash_image, image);
> +	}
> +	arch_unmap_crash_pages((void **)&ptr);
> +	pr_debug("crash hp: re-loaded elfcorehdr at 0x%lx\n", mem);
> +
> +out:
> +	if (elfbuf)
> +		vfree(elfbuf);
> +}
> +#endif
> --=20
> 2.31.1
>=20
>=20