Re: [PATCH] kdump, x86: report actual value of phys_base in VMCOREINFO

[HATAYAMA Daisuke]

From: Petr Tesarik <ptesarik@xxxxxxx>
Subject: Re: [PATCH] kdump, x86: report actual value of phys_base in VMCOREINFO
Date: Wed, 12 Nov 2014 09:14:34 +0100

> On Wed, 12 Nov 2014 15:40:42 +0900 (JST)
> HATAYAMA Daisuke <d.hatayama@xxxxxxxxxxxxxx> wrote:
> 
>> Currently, VMCOREINFO note information reports the virtual address of
>> phys_base that is assigned to symbol phys_base. But this doesn't make
>> sense because to refer to value of the phys_base, it's necessary to
>> get the value of phys_base itself we are now about to refer to.
>> 
>> Userland tools related to kdump such as makedumpfile and crash utility
>> so far have made some efforts to calculate phys_base from memory
>> mapping information on a variety of crash dump formats. But there's no
>> guarantee to keep maintaining it in the future.
>> 
>> This is also useful for crash dump mechanism running outside Linux
>> kernel such as virtual machine hypervisor such as qemu dump, which
>> ordinary users use via virsh dump, or ones implemented on vendor
>> specific firmware. They cannot get phys_base without special mechanism
>> because phys_base is kernel information.
>> 
>> To get VMCOREINFO in vmcore, it's easy to use strings and grep
>> commands like this; VMCOREINFO consists of simple string:
>> 
>> $ strings vmcore-3.10.0-121.el7.x86_64 | grep -E ".*VMCOREINFO.*" -A 100
>> VMCOREINFO
>> OSRELEASE=3.10.0-121.el7.x86_64
>> PAGESIZE=4096
>> ...
>> 
>> Similarly, this is also useful to get value of phys_base in kdump 2nd
>> kernel contained in vmcore using the above-mentioned external crash
>> dump mechanism; kdump 2nd kernel is an inherently relocated kernel.
>> 
>> This commit doesn't remove VMCOREINFO_SYMBOL(phys_base) line because
>> makedumpfile refers to it and if removing it, old versions
>> makedumpfile doesn't work well.
>> 
>> Signed-off-by: HATAYAMA Daisuke <d.hatayama@xxxxxxxxxxxxxx>
>> ---
>>  arch/x86/kernel/machine_kexec_64.c | 1 +
>>  1 file changed, 1 insertion(+)
>> 
>> diff --git a/arch/x86/kernel/machine_kexec_64.c b/arch/x86/kernel/machine_kexec_64.c
>> index 4859810..e6d00a4 100644
>> --- a/arch/x86/kernel/machine_kexec_64.c
>> +++ b/arch/x86/kernel/machine_kexec_64.c
>> @@ -334,6 +334,7 @@ void arch_crash_save_vmcoreinfo(void)
>>  #endif
>>  	vmcoreinfo_append_str("KERNELOFFSET=%lx\n",
>>  			      (unsigned long)&_text - __START_KERNEL);
>> +	VMCOREINFO_LENGTH(phys_base, phys_base);
> 
> While I fully agree with the concept, I don't like the use of
> VMCOREINFO_LENGTH. LENGTH(symbol) has been used to store array length
> in VMCOREINFO.
> 
> OTOH there is currently no good syntax for storing a value (short of
> VMCOREINFO_NUMBER, but that one is signed). I think it would be best to
> extend the VMCOREINFO syntax for storing variables, preferably in
> hexadecimal, e.g.:
> 
> #define VMCOREINFO_VALUE(name, value) \
> 	vmcoreinfo_append_str("VALUE(%s)=%lx\n", #name, (unsigned long) value)
> 
> This interface is somewhat suboptimal, because it can only store a
> single long value. So, maybe we should dump the complete variable in
> hex, or something similar...
> 
> Anyone has a better idea?
> 
> Petr T

Just as you say, it's natural to write value of phys_base in
hexadecimal format.

For VMCOREINFO_VALUE, looking at the current helper macros, it seems
to me more natural to make it have more specific name:

#define VMCOREINFO_PHYS_BASE(value) \
	vmcoreinfo_append_str("PHYS_BASE=%lx\n", (unsigned long) value)

--
Thanks.
HATAYAMA, Daisuke

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