[PATCH Makedumpfile 1/4] x86_64: Calculate page_offset from pt_load

[Atsushi Kumagai]

Hello,

>On 11/02/16 at 07:40am, Atsushi Kumagai wrote:
>> Hello Pratyush,
>>
>> >On Monday 24 October 2016 10:18 PM, Pratyush Anand wrote:
>> >> page_offset can always be calculated as 'virtual - physical' for a direct
>> >> mapping area on x86. Therefore, remove the version dependent calculation
>> >> and use this method.
>> >>
>> >> Signed-off-by: Pratyush Anand <panand at redhat.com>
>> >> ---
>> >>  arch/x86_64.c | 24 ++++++++++++++++++++----
>> >>  1 file changed, 20 insertions(+), 4 deletions(-)
>> >>
>> >> diff --git a/arch/x86_64.c b/arch/x86_64.c
>> >> index ddf7be6bc57b..a96fd8ae00a1 100644
>> >> --- a/arch/x86_64.c
>> >> +++ b/arch/x86_64.c
>> >> @@ -44,6 +44,24 @@ get_xen_p2m_mfn(void)
>> >>  	return NOT_FOUND_LONG_VALUE;
>> >>  }
>> >>
>> >> +static int
>> >> +get_page_offset_x86_64(void)
>> >> +{
>> >> +	int i;
>> >> +	unsigned long long phys_start;
>> >> +	unsigned long long virt_start;
>> >> +
>> >> +	for (i = 0; get_pt_load(i, &phys_start, NULL, &virt_start, NULL); i++) {
>> >> +		if (virt_start >= __START_KERNEL_map) {
>> >
>> >OK..So, this is the problem. We should have
>> >
>> >if (virt_start < __START_KERNEL_map) {
>> >
>> >Kernel text region lies above __START_KERNEL_map, which is linearly
>> >mapped however not a direct mapping. Direct mapping region lies below it
>> >instead. So, page_offset can only be calculated with a region which is
>> >below __START_KERNEL_map.
>> >
>> >Thanks Baoquan for finding it.
>>
>> Could you explain the issue in more detail, what is the difference
>> between "linear map" and "direct map" ?
>
>Well, these could be easily misunderstood. Virtual address in kernel text
>region or within direct mapping which is from PAGE_OFFSET are all linear
>mapping. Because the related PA can be calculated by:
>PA= VA - PAGE_OFFSET; //y=x - a;
>PA= VA - __START_KERNEL_map + phys_base; //y=x-a+b
>
>In user-space kexec-tools we created pt_load program segments for kernel
>text region and crash memory regions. The first program header is kernel
>text region. So here if we want to get page_offset, just need to ignore
>the 1st one, kernel text region.

Thanks, I understand.
Now I have no concerns.

Regards,
Atsushi Kumagai

>> BTW, I confirmed that the difference of v2 patch is only this and
>> the result of benchmark looks great, thank you very much.
>>
>>
>> Regards,
>> Atsushi Kumagai
>>
>> >> +			info->page_offset = virt_start - phys_start;
>> >> +			return TRUE;
>> >> +		}
>> >> +	}
>> >> +
>> >> +	ERRMSG("Can't get any pt_load to calculate page offset.\n");
>> >> +	return FALSE;
>> >> +}
>> >> +
>> >>  int
>> >>  get_phys_base_x86_64(void)
>> >>  {
>> >> @@ -159,10 +177,8 @@ get_versiondep_info_x86_64(void)
>> >>  	else
>> >>  		info->max_physmem_bits  = _MAX_PHYSMEM_BITS_2_6_31;
>> >>
>> >> -	if (info->kernel_version < KERNEL_VERSION(2, 6, 27))
>> >> -		info->page_offset = __PAGE_OFFSET_ORIG;
>> >> -	else
>> >> -		info->page_offset = __PAGE_OFFSET_2_6_27;
>> >> +	if (!get_page_offset_x86_64())
>> >> +		return FALSE;
>> >>
>> >>  	if (info->kernel_version < KERNEL_VERSION(2, 6, 31)) {
>> >>  		info->vmalloc_start = VMALLOC_START_ORIG;
>> >>
>> >
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>
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