[PATCH] arm64, vmcoreinfo : Append 'MAX_USER_VA_BITS' and 'MAX_PHYSMEM_BITS' to vmcoreinfo
bhsharma at redhat.com
Mon Feb 4 06:35:41 PST 2019
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 .
>> Why does it need to know?
>> (Suzuki asked the same question on your earlier version)
> 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
> 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..>
> And the makedumpfile code uses the same to determine support for 5-level
> page tables in x86_64, see
> 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.
>>> diff --git a/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(MAX_USER_VA_BITS);
>>> + VMCOREINFO_NUMBER(MAX_PHYSMEM_BITS);
>>> /* Please note VMCOREINFO_NUMBER() uses "%d", not "%x" */
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