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

From: Dave Anderson <anderson@redhat.com>
To: Bhupesh Sharma <bhsharma@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>,
	Kazuhito Hagio <k-hagio@ab.jp.nec.com>,
	lijiang@redhat.com, bhe@redhat.com,
	ard biesheuvel <ard.biesheuvel@linaro.org>,
	catalin marinas <catalin.marinas@arm.com>,
	Steve Capper <Steve.Capper@arm.com>,
	kexec@lists.infradead.org, Will Deacon <will.deacon@arm.com>,
	AKASHI Takahiro <takahiro.akashi@linaro.org>,
	James Morse <james.morse@arm.com>,
	Kristina Martsenko <kristina.martsenko@arm.com>,
	Borislav Petkov <bp@alien8.de>, Dave Young <dyoung@redhat.com>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH] arm64, vmcoreinfo : Append 'MAX_USER_VA_BITS' and 'MAX_PHYSMEM_BITS' to vmcoreinfo
Date: Thu, 21 Feb 2019 11:42:29 -0500 (EST)	[thread overview]
Message-ID: <1481013752.3226345.1550767349644.JavaMail.zimbra@redhat.com> (raw)
In-Reply-To: <891eaf5a-aede-364d-6465-832e377c3e29@redhat.com>

----- 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.

Dave

> Thanks,
> Bhupesh
> 
> 

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