From: Bhupesh Sharma <bhsharma@redhat.com>
To: Dave Young <dyoung@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@linaro.org,
catalin.marinas@arm.com, kexec@lists.infradead.org,
Will Deacon <will.deacon@arm.com>,
AKASHI Takahiro <takahiro.akashi@linaro.org>,
James Morse <james.morse@arm.com>, Borislav Petkov <bp@alien8.de>,
anderson@redhat.com, bhupesh.linux@gmail.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, 31 Jan 2019 15:30:23 +0530 [thread overview]
Message-ID: <fea7823d-60ab-2765-031f-6e8c312d7672@redhat.com> (raw)
In-Reply-To: <20190131014800.GB15785@dhcp-128-65.nay.redhat.com>
On 01/31/2019 07:18 AM, 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.
I explained this a bit in my reply to Suzuki's and James's review
comments yesterday, but let me summarize the same again for better clarity:
Let's take the example of x86. We have CONFIG_X86_5LEVEL config flag to
indicate 5 level page-table support. We export the same in vmcoreinfo
for x86_64 using:
void arch_crash_save_vmcoreinfo(void)
{
<.. snip..>
vmcoreinfo_append_str("NUMBER(pgtable_l5_enabled)=%d\n",
pgtable_l5_enabled());
}
Also a simple grep in makedumpfile and crash for the same indicates that
the user-space code determines the MAX_PHYSMEM_BITS value using the
'pgtable_l5_enabled' value available in vmcoreinfo:
Example from makedumpfile:
-------------------------
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;
...
}
As we can see above, we use several if-else cases to determine the
'MAX_PHYSMEM_BITS', one of which is setting it to 52-bit, if the
'pgtable_l5_enabled' value is available and TRUE in vmcoreinfo.
So, we determine 'MAX_PHYSMEM_BITS' value in makedumpfile via a
vmcoreinfo export'ed variable (rather than using named
'MAX_PHYSMEM_BITS' its 'pgtable_l5_enabled' that is being used).
Since for arm64, we don't have a single CONFIG flag for 52-bit addresses
spaces (kernel VA, user-space VA and PA), so its better to export the
respective CONFIG flags in the vmcoreinfo directly.
> If we really need it then should it be arm64 only?
See above, since archs like x86 use a single flag: CONFIG_X86_5LEVEL,
whereas arm64 can use the combination of following flags to indicate
combinations of various address spaces:
- 48-bit kernel VA + 48-bit user-space VA + 52-bit PA
- 48-bit kernel VA + 52-bit user-space VA + 52-bit PA
- 52-bit kernel VA + 52-bit user-space VA + 52-bit PA
CONFIG_ARM64_64K_PAGES
CONFIG_ARM64_USER_VA_BITS_52
CONFIG_ARM64_VA_BITS
CONFIG_ARM64_PA_BITS_52
CONFIG_ARM64_PA_BITS
CONFIG_EXPERT, and
CONFIG_ARM64_FORCE_52BIT
so probably its not correct to compare the two cases one-to-one (its
more like an apple and an orange comparison).
> If it is arm64 only then the makedumpfile code should read this number
> only for arm64.
>
> Also Lianbo added the vmcoreinfo documents, I believe it stays in -tip
> tree, need to make sure to document this as well.
Sure, I will send a separate patch to fix the same, once this gets in.
>
> 2. MAX_USER_VA_BITS
> Does makedumpfile care about userspace VA bits?
Yes. Consider the case 48-bit kernel VA and 52-bit user-space VA, which
is a perfectly valid case on arm64. In such cases VA_BITS is set to 48,
whereas MAX_USER_VA_BITS is set to 52, which allows user-space
applications which use 52-bit virtual address to pass a hint to 'mmap'
to get high addresses.
I do not see other code
> doing this, Kazu and Dave A should be able to comment.
I talked to Dave A. yesterday off-list, I think he mentioned that these
changes are useful for crash-utility as well and he was hoping it gets
accepted soon so that kernel-debugging tools can handle increased
address spaces on arm64.
But it will be great to have reviews/ACKs from Dave A and others as well.
Thanks,
Bhupesh
>
> I tend to doubt about this.
>
>>
>> Cc: AKASHI Takahiro <takahiro.akashi@linaro.org>
>> Cc: Mark Rutland <mark.rutland@arm.com>
>> Cc: Will Deacon <will.deacon@arm.com>
>> Cc: James Morse <james.morse@arm.com>
>> Signed-off-by: Bhupesh Sharma <bhsharma@redhat.com>
>> ---
>> arch/arm64/kernel/crash_core.c | 2 ++
>> 1 file changed, 2 insertions(+)
>>
>> 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);
>> --
>> 2.7.4
>>
>>
>> _______________________________________________
>> kexec mailing list
>> kexec@lists.infradead.org
>> http://lists.infradead.org/mailman/listinfo/kexec
>
> Thanks
> Dave
>
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next prev parent reply other threads:[~2019-01-31 10:00 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-01-30 12:23 [PATCH] arm64, vmcoreinfo : Append 'MAX_USER_VA_BITS' and 'MAX_PHYSMEM_BITS' to vmcoreinfo Bhupesh Sharma
2019-01-30 15:21 ` James Morse
2019-01-30 21:39 ` Bhupesh Sharma
2019-02-04 14:35 ` Bhupesh Sharma
2019-02-04 15:31 ` Robin Murphy
2019-02-12 4:55 ` Bhupesh Sharma
2019-02-12 10:49 ` Robin Murphy
2019-02-04 16:56 ` James Morse
2019-01-31 1:48 ` Dave Young
2019-01-31 10:00 ` Bhupesh Sharma [this message]
2019-01-31 14:03 ` Dave Anderson
2019-02-04 16:04 ` Kazuhito Hagio
2019-02-12 5:07 ` Bhupesh Sharma
2019-02-12 10:44 ` Dave Young
2019-02-12 19:59 ` Bhupesh Sharma
2019-02-12 23:03 ` Kazuhito Hagio
2019-02-13 11:15 ` Dave Young
2019-02-13 18:22 ` James Morse
2019-02-13 19:52 ` Kazuhito Hagio
2019-02-15 17:34 ` James Morse
2019-02-15 18:01 ` Bhupesh Sharma
2019-02-18 15:27 ` Steve Capper
2019-02-21 16:08 ` Bhupesh Sharma
2019-02-19 20:47 ` Kazuhito Hagio
2019-02-21 16:20 ` Bhupesh Sharma
2019-02-21 16:42 ` Dave Anderson
2019-02-21 19:02 ` Kazuhito Hagio
2019-03-01 4:01 ` Bhupesh Sharma
2019-02-14 19:30 ` Bhupesh Sharma
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