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

[Bhupesh Sharma <bhsharma@redhat.com>]

Hi Kazu,

On 02/04/2019 09:34 PM, Kazuhito Hagio wrote:
> On 1/30/2019 8:48 PM, Dave Young wrote:
>> + more people
>> On 01/30/19 at 05:53pm, 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].
>>>
>>> [0].
>> https://github.com/bhupesh-sharma/makedumpfile/blob/52-bit-pa-support-via-vmcore-v1/arch/arm64.c#L490
>>
>> I'm not objecting the patch, just want to make sure to make clear about
>> things and make sure these issues are aware by people, and leave arm
>> people to review the arm bits.
>>
>> 1. MAX_PHYSMEM_BITS
>> As we previously found, back to 2014 makedumpfile took a patch to read the
>> value from vmcore but the kernel patch was not accepted.
>> So we should first make clear if this is really needed, why other arches
>> do not need this in makedumpfile.
>>
>> If we really need it then should it be arm64 only?
>>
>> If it is arm64 only then the makedumpfile code should read this number
>> only for arm64.
> 
> Sorry for the delay.
> 
> According to the kernel patch, some of arm32 platforms may need it
> http://lists.infradead.org/pipermail/kexec/2014-May/011909.html
> but except for them (and arm64), makedumpfile can manage with kernel
> version and some switches to determine this value so far.
> 
>>
>> Also Lianbo added the vmcoreinfo documents, I believe it stays in -tip
>> tree,  need to make sure to document this as well.
>>
>> 2. MAX_USER_VA_BITS
>> Does makedumpfile care about userspace VA bits?  I do not see other code
>> doing this,  Kazu and Dave A should be able to comment.
> 
> The mapping makedumpfile uses on arm64 is swapper_pg_dir only, so
> unless the config affects its structure or something, makedumpfile
> will not need this value.

I captured this case in more details while sending out the makedumpfile  
enablement patch for ARMv8.2-LVA (see [0]), but here is a brief summary  
on the same:

Since at the moment we enable the support of the ARMv8.2-LVA extension  
for 52-bit user-space VA in the kernel via a CONFIG flags  
(CONFIG_ARM64_USER_VA_BITS_52), so there are no clear mechanisms in  
user-space to determine this CONFIG
flag value and use it to determine the address range values.

Since 'VA_BITS' are already exported via vmcoreinfo, if we export  
'MAX_USER_VA_BITS' as well, we can use the same in user-space to check  
if the 'MAX_USER_VA_BITS' value is greater than 'VA_BITS'. If yes, then  
we are running a use-case where user-space is 52-bit while the  
underlying kernel is still 48-bit.

The increased 'PTRS_PER_PGD' value for such cases needs to be then  
calculated as is done by the underlying kernel (see  
'arch/arm64/include/asm/pgtable-hwdef.h' for details):

#define PTRS_PER_PGD		(1 << (MAX_USER_VA_BITS - PGDIR_SHIFT))

Also, note that 'arch/arm64/include/asm/memory.h' defines  
'MAX_USER_VA_BITS' as 'VA_BITS' in case 'CONFIG_ARM64_USER_VA_BITS_52'  
is set to 'n':

#ifdef CONFIG_ARM64_USER_VA_BITS_52
#define MAX_USER_VA_BITS	52
#else
#define MAX_USER_VA_BITS	VA_BITS
#endif

So, makedumpfile will need this symbol exported in vmcore to make the  
above determination.

[0]. http://lists.infradead.org/pipermail/kexec/2019-February/022425.html

Thanks,
Bhupesh

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