Subject: [PATCH v12 13/16] arm64: kdump: add kdump support

From: [email-masked] (AKASHI Takahiro)
Date: Wed, 16 Dec 2015 14:41:35 +0900

On 12/16/2015 02:45 AM, Will Deacon wrote:
> On Tue, Nov 24, 2015 at 10:25:34PM +0000, Geoff Levand wrote:
>> From: AKASHI Takahiro <[email-masked]>
>>
>> On crash dump kernel, all the information about primary kernel's core
>> image is available in elf core header specified by "elfcorehdr=" boot
>> parameter. reserve_elfcorehdr() will set aside the region to avoid any
>> corruption by crash dump kernel.
>>
>> Crash dump kernel will access the system memory of primary kernel via
>> copy_oldmem_page(), which reads one page by ioremap'ing it since it does
>> not reside in linear mapping on crash dump kernel.
>> Please note that we should add "mem=X[MG]" boot parameter to limit the
>> memory size and avoid the following assertion at ioremap():
>> 	if (WARN_ON(pfn_valid(__phys_to_pfn(phys_addr))))
>> 		return NULL;
>> when accessing any pages beyond the usable memories of crash dump kernel.
>>
>> We also need our own elfcorehdr_read() here since the weak definition of
>> elfcorehdr_read() utilizes copy_oldmem_page() and will hit the assertion
>> above on arm64.
>>
>> Signed-off-by: AKASHI Takahiro <[email-masked]>
>> ---
>>   arch/arm64/Kconfig             | 12 +++++++
>>   arch/arm64/kernel/Makefile     |  1 +
>>   arch/arm64/kernel/crash_dump.c | 71 ++++++++++++++++++++++++++++++++++++++++++
>>   arch/arm64/mm/init.c           | 29 +++++++++++++++++
>>   4 files changed, 113 insertions(+)
>>   create mode 100644 arch/arm64/kernel/crash_dump.c
>>
>> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
>> index c23fd77..4bac7dc 100644
>> --- a/arch/arm64/Kconfig
>> +++ b/arch/arm64/Kconfig
>> @@ -545,6 +545,18 @@ config KEXEC
>>   	  but it is independent of the system firmware.   And like a reboot
>>   	  you can start any kernel with it, not just Linux.
>>
>> +config CRASH_DUMP
>> +	bool "Build kdump crash kernel"
>> +	help
>> +	  Generate crash dump after being started by kexec. This should
>> +	  be normally only set in special crash dump kernels which are
>> +	  loaded in the main kernel with kexec-tools into a specially
>> +	  reserved region and then later executed after a crash by
>> +	  kdump/kexec. The crash dump kernel must be compiled to a
>> +	  memory address not used by the main kernel.
>
> What does this even mean? How do I "compile to a memory address not used
> by the main kernel"?

Well, it's just a copy from arm, but right, it's ambiguous.
I will remove that text.

>> diff --git a/arch/arm64/kernel/crash_dump.c b/arch/arm64/kernel/crash_dump.c
>> new file mode 100644
>> index 0000000..3d86c0a
>> --- /dev/null
>> +++ b/arch/arm64/kernel/crash_dump.c
>> @@ -0,0 +1,71 @@
>> +/*
>> + * Routines for doing kexec-based kdump
>> + *
>> + * Copyright (C) 2014 Linaro Limited
>> + * Author: AKASHI Takahiro <[email-masked]>
>> + *
>> + * This program is free software; you can redistribute it and/or modify
>> + * it under the terms of the GNU General Public License version 2 as
>> + * published by the Free Software Foundation.
>> + */
>> +
>> +#include <linux/crash_dump.h>
>> +#include <linux/errno.h>
>> +#include <linux/io.h>
>> +#include <linux/memblock.h>
>> +#include <linux/uaccess.h>
>> +#include <asm/memory.h>
>> +
>> +/**
>> + * copy_oldmem_page() - copy one page from old kernel memory
>> + * @pfn: page frame number to be copied
>> + * @buf: buffer where the copied page is placed
>> + * @csize: number of bytes to copy
>> + * @offset: offset in bytes into the page
>> + * @userbuf: if set, @buf is in a user address space
>> + *
>> + * This function copies one page from old kernel memory into buffer pointed by
>> + * @buf. If @buf is in userspace, set @userbuf to %1. Returns number of bytes
>> + * copied or negative error in case of failure.
>> + */
>> +ssize_t copy_oldmem_page(unsigned long pfn, char *buf,
>> +			 size_t csize, unsigned long offset,
>> +			 int userbuf)
>> +{
>> +	void *vaddr;
>> +
>> +	if (!csize)
>> +		return 0;
>> +
>> +	vaddr = ioremap_cache(pfn << PAGE_SHIFT, PAGE_SIZE);
>
> pfn_to_page

Maybe __pfn_to_phsy()?

>> +	if (!vaddr)
>> +		return -ENOMEM;
>> +
>> +	if (userbuf) {
>> +		if (copy_to_user(buf, vaddr + offset, csize)) {
>> +			iounmap(vaddr);
>> +			return -EFAULT;
>> +		}
>> +	} else {
>> +		memcpy(buf, vaddr + offset, csize);
>> +	}
>> +
>> +	iounmap(vaddr);
>> +
>> +	return csize;
>> +}
>> +
>> +/**
>> + * elfcorehdr_read - read from ELF core header
>> + * @buf: buffer where the data is placed
>> + * @csize: number of bytes to read
>> + * @ppos: address in the memory
>> + *
>> + * This function reads @count bytes from elf core header which exists
>> + * on crash dump kernel's memory.
>> + */
>> +ssize_t elfcorehdr_read(char *buf, size_t count, u64 *ppos)
>> +{
>> +	memcpy(buf, phys_to_virt((phys_addr_t)*ppos), count);
>> +	return count;
>> +}
>
> I know you say that we have to override this function so that we don't
> hit the pfn_valid warning in ioremap, but what guarantees that the ELF
> header of the crashed kernel is actually mapped in our linear mapping?

Well, in fact, it depends on kexec-tools.
In the current implementation for arm64, the elf core header is allocated
within the usable memory of crash dump kernel.

Should we add some check here?

>> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
>> index 24f0a1c..52a1469 100644
>> --- a/arch/arm64/mm/init.c
>> +++ b/arch/arm64/mm/init.c
>> @@ -35,6 +35,7 @@
>>   #include <linux/efi.h>
>>   #include <linux/swiotlb.h>
>>   #include <linux/kexec.h>
>> +#include <linux/crash_dump.h>
>>
>>   #include <asm/fixmap.h>
>>   #include <asm/memory.h>
>> @@ -116,6 +117,31 @@ static void __init reserve_crashkernel(void)
>>   }
>>   #endif /* CONFIG_KEXEC */
>>
>> +#ifdef CONFIG_CRASH_DUMP
>> +/*
>> + * reserve_elfcorehdr() - reserves memory for elf core header
>> + *
>> + * This function reserves elf core header given in "elfcorehdr=" kernel
>> + * command line parameter. This region contains all the information about
>> + * primary kernel's core image and is used by a dump capture kernel to
>> + * access the system memory on primary kernel.
>> + */
>> +static void __init reserve_elfcorehdr(void)
>> +{
>> +	if (!elfcorehdr_size)
>> +		return;
>> +
>> +	if (memblock_is_region_reserved(elfcorehdr_addr, elfcorehdr_size)) {
>> +		pr_warn("elfcorehdr is overlapped\n");
>> +		return;
>> +	}
>> +
>> +	memblock_reserve(elfcorehdr_addr, elfcorehdr_size);
>> +
>> +	pr_info("Reserving %lldKB of memory at %lldMB for elfcorehdr\n",
>> +		elfcorehdr_size >> 10, elfcorehdr_addr >> 20);
>
> I'd have thought it would be more useful to print the address as an
> address rather than a size.

Yeah, I totally agree, but all the other archs, including x86 and arm,
print the address in "%lldMB" format.
If you like, I can fix it.

>> +}
>
> Similar #else trick here.

Sure.

Thanks,
-Takahiro AKASHI

> Will