RE: [PATCH] arm64, vmcoreinfo : Append 'MAX_USER_VA_BITS' and 'MAX_PHYSMEM_BITS' to vmcoreinfo

[Kazuhito Hagio <k-hagio@ab.jp.nec.com>]

-----Original Message-----
> ----- Original Message -----
> > Hi Kazu,
> >
> > On 02/20/2019 02:17 AM, Kazuhito Hagio wrote:
> > > Hi Bhupesh,
> > >
> > > -----Original Message-----
> > >> I am not sure you got a chance to look at the two regression cases I
> > >> reported here:
> > >> <http://lists.infradead.org/pipermail/kexec/2019-February/022449.html>
> > >>
> > >> Unfortunately the above suggestion doesn't provide any fix for
> > >> ARMv8.2-LPA regression (see text under heading '
> > >> (1). Regression Case 1 (ARMv8.2-LPA enabled kernel)')
> > >
> > > As for MAX_PHYSMEM_BITS, I realized that ppc64 makedumpfile can detect
> > > it because there is only one SECTION_SIZE_BITS for ppc64. I think we
> > > can use the same way as set_ppc64_max_physmem_bits() does also for
> > > arm64 for now. I'm going to write it for kernels not having
> > > NUMBER(MAX_PHYSMEM_BITS) in vmcoreinfo.
> >
> > I see two drawbacks with the above approach:
> >
> > a). This means that other user-space tools like crash-utility would
> > still be broken and would probably need to find MAX_PHYSMEM_BITS for
> > arm64 via a similar (hack'ish ?) approach.
> >
> > b). I am looking at the makedumpfile code for 'MAX_PHYSMEM_BITS'
> > determination for two archs as an example:
> >
> > ppc
> > ---
> >
> > int
> > set_ppc64_max_physmem_bits(void)
> > {
> >          long array_len = ARRAY_LENGTH(mem_section);
> >          /*
> >           * The older ppc64 kernels uses _MAX_PHYSMEM_BITS as 42 and the
> >           * newer kernels 3.7 onwards uses 46 bits.
> >           */
> >
> >          info->max_physmem_bits  = _MAX_PHYSMEM_BITS_ORIG ;
> >          if ((array_len == (NR_MEM_SECTIONS() / _SECTIONS_PER_ROOT_EXTREME()))
> >                  || (array_len == (NR_MEM_SECTIONS() / _SECTIONS_PER_ROOT())))
> >                  return TRUE;
> >
> >          info->max_physmem_bits  = _MAX_PHYSMEM_BITS_3_7;
> >          if ((array_len == (NR_MEM_SECTIONS() / _SECTIONS_PER_ROOT_EXTREME()))
> >                  || (array_len == (NR_MEM_SECTIONS() / _SECTIONS_PER_ROOT())))
> >                  return TRUE;
> >
> >          info->max_physmem_bits  = _MAX_PHYSMEM_BITS_4_19;
> >          if ((array_len == (NR_MEM_SECTIONS() / _SECTIONS_PER_ROOT_EXTREME()))
> >                  || (array_len == (NR_MEM_SECTIONS() / _SECTIONS_PER_ROOT())))
> >                  return TRUE;
> >
> >          info->max_physmem_bits  = _MAX_PHYSMEM_BITS_4_20;
> >          if ((array_len == (NR_MEM_SECTIONS() / _SECTIONS_PER_ROOT_EXTREME()))
> >                  || (array_len == (NR_MEM_SECTIONS() /  _SECTIONS_PER_ROOT())))
> >                  return TRUE;
> >
> >          return FALSE;
> > }
> >
> > x86_64:
> > ------
> >
> > int
> > get_versiondep_info_x86_64(void)
> > {
> >      /*
> >       * On linux-2.6.26, MAX_PHYSMEM_BITS is changed to 44 from 40.
> >       */
> >      if (info->kernel_version < KERNEL_VERSION(2, 6, 26))
> >          info->max_physmem_bits  = _MAX_PHYSMEM_BITS_ORIG;
> >      else if (info->kernel_version < KERNEL_VERSION(2, 6, 31))
> >          info->max_physmem_bits  = _MAX_PHYSMEM_BITS_2_6_26;
> >      else if(check_5level_paging())
> >          info->max_physmem_bits  = _MAX_PHYSMEM_BITS_5LEVEL;
> >      else
> >          info->max_physmem_bits  = _MAX_PHYSMEM_BITS_2_6_31;
> >
> >      ...
> > }
> >
> > Looking at the above, two questions come to my mind:
> >
> > - Do we really need all the above complexity in user-space code, to hoop
> > across various kernel versions and perform allocations for something
> > that can be so easily exported via vmcoreinfo? Also we need to see how
> > portable is the above code for a new kernel version - IMO, it will need
> > another fix patch when we update to a new kernel version in near future.
> 
> I agree -- not to mention that the "kernel version" way of determining things
> does not account for distribution-specific backports.
> 
> >
> > - Also do we need to replicate the above implementations across
> > user-space tools when they can also utilize the vmcoreinfo information
> > to determine the PA_BITS range without any additional arch/kernel
> > version specific details as the single point of obtaining this
> > information from the kernel?
> >
> > So, in view of the above, I would still advocate that we use a
> > vmcoreinfo export for 'MAX_PHYSMEM_BITS' as well to have a uniform
> > interface for the same across all user-land applications.
> 
> Again, totally agree.

I also agree that we should do so. Then it will be better to have
it in kernel core code, not in arch-specific code.

Although makedumpfile may have to have the kludge for kernels that
support 52-bit PA and don't have the exported MAX_PHYSMEM_BITS
sooner or later..

Thanks,
Kazu


> 
> Dave
> 
> 
> > Thanks,
> > Bhupesh
> >
> >
> 
> _______________________________________________
> kexec mailing list
> kexec@lists.infradead.org
> http://lists.infradead.org/mailman/listinfo/kexec



_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel