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

[Bhupesh Sharma <bhsharma@redhat.com>]

Hi James, All,

On 01/31/2019 03:09 AM, Bhupesh Sharma wrote:
> Hi James,
> 
> Thanks for review.
> Please see my comments inline.
> 
> 
> On 01/30/2019 08:51 PM, James Morse wrote:
>> Hi Bhupesh,
>>
>> On 01/30/2019 12:23 PM, Bhupesh Sharma wrote:
>>> With ARMv8.2-LVA and LPA architecture extensions, arm64 hardware which
>>> supports these extensions can support upto 52-bit virtual and 52-bit
>>> physical addresses respectively.
>>>
>>> Since at the moment we enable the support of these extensions via CONFIG
>>> flags, e.g.
>>>   - LPA via CONFIG_ARM64_PA_BITS_52
>>>
>>> there are no clear mechanisms in user-space right now to
>>> deteremine these CONFIG flag values and also determine the PARange and
>>> VARange address values.
>>> User-space tools like 'makedumpfile' and 'crash-utility' can instead
>>> use the 'MAX_USER_VA_BITS' and 'MAX_PHYSMEM_BITS' values to determine
>>> the maximum virtual address and physical address (respectively)
>>> supported by underlying kernel.
>>>
>>> A reference 'makedumpfile' implementation which uses this approach to
>>> determining the maximum physical address is available in [0].
>>
>> Why does it need to know?
>>
>> (Suzuki asked the same question on your earlier version)
>> https://lore.kernel.org/linux-arm-kernel/cff44754-7fe4-efea-bc8e-4dde2277c821@arm.com/ 
> 
> 
> I have shared some details (after discussion with our test teams) in 
> reply to the review comments from Suzuki here:
> http://lists.infradead.org/pipermail/kexec/2019-January/022389.html, and
> http://lists.infradead.org/pipermail/kexec/2019-January/022390.html
> 
> Just to summarize, I mentioned in my replies to the review comments tha 
> the makedumpfile implementation (for decoding the PTE) was just as an 
> example, however there can be other user-space applications, for e.g a 
> user-space application running with 48-bit kernel VA and 52-bit user 
> space VA and requesting allocation in 'high' address via a 'hint' to mmap.
> 
>>  From your github link it looks like you use this to re-assemble the 
>> two bits of the PFN from the pte. Can't you always do this for 64K 
>> pages? CPUs with the feature always do this too, its not something the 
>> kernel turns on.
> 
> Ok, let me try to give some perspective of a common makedumpfile 
> use-case before I jump into the details:
> 
> (a) makedumpfile tool can be used to generate a vmcore and analyze it 
> later. So for example we can create vmcore for a system running with 
> page-size = 64K and analyze it later on a different system using 
> page-size = 4K.
> 
> Since several makedumpfile code legs (for page-table walk) are common in 
> both the paths (creating a vmcore and analyzing a vmcore), we cannot 
> hardcode the PTE calculation masks for either 48-bit or 52-bit address 
> spaces (or 4K/64K page sizes). The example invocations for the two cases 
> is given below:
> 
> Create a vmcore dump on a 64K machine:
> # makedumpfile -l --message-level 1 -d 31 /proc/vmcore vmcore
> 
> Analyze the vmcore dump on a 4K machine:
> # makedumpfile -d 31 -x vmlinux vmcore dumpfile
> 
> Also hardcoding the PTE calculation to use the high address bit mask 
> always will break the backward compatibility with older kernels (which 
> don't support 52-bit address space extensions).
> 
> (b). Also x86_64 already has a vmcoreinfo export for 'pgtable_l5_enabled':
> 
> void arch_crash_save_vmcoreinfo(void)
> {
>      <.. snip..>
>      vmcoreinfo_append_str("NUMBER(pgtable_l5_enabled)=%d\n",
>              pgtable_l5_enabled());
> }
> 
> And the makedumpfile code uses the same to determine support for 5-level 
> page tables in x86_64, see 
> <https://github.com/bhupesh-sharma/makedumpfile/blob/52-bit-pa-support-via-vmcore-v1/arch/x86_64.c#L36> 
> for example.

Ping. Since this patch fixes a regression with user-space tools like 
makedumpfile and crash-utility which are broken since arm64 kernels 
with 52-bit VA and PA support are available (and distributions which 
enable them), would request review comments/ack on this simple change.

Thanks,
Bhupesh

>>
>>> diff --git a/arch/arm64/kernel/crash_core.c 
>>> b/arch/arm64/kernel/crash_core.c
>>> index ca4c3e12d8c5..ad231be5c0d8 100644
>>> --- a/arch/arm64/kernel/crash_core.c
>>> +++ b/arch/arm64/kernel/crash_core.c
>>> @@ -10,6 +10,8 @@
>>>   void arch_crash_save_vmcoreinfo(void)
>>>   {
>>>       VMCOREINFO_NUMBER(VA_BITS);
>>> +    VMCOREINFO_NUMBER(MAX_USER_VA_BITS);
>>> +    VMCOREINFO_NUMBER(MAX_PHYSMEM_BITS);
>>>       /* Please note VMCOREINFO_NUMBER() uses "%d", not "%x" */
>>>       vmcoreinfo_append_str("NUMBER(kimage_voffset)=0x%llx\n",
>>>                           kimage_voffset);
>>>
>>
> 


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