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

[bhe at redhat.com]

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.

> 
> 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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