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Subject: Re: [PATCH 2/2 v5] x86/kexec_file: add reserved e820 ranges to kdump
 kernel e820 table
To:     Baoquan He <bhe@redhat.com>
Cc:     linux-kernel@vger.kernel.org, kexec@lists.infradead.org,
        x86@kernel.org, tglx@linutronix.de, mingo@redhat.com, bp@alien8.de,
        akpm@linux-foundation.org, dyoung@redhat.com
References: <20181107050019.6663-1-lijiang@redhat.com>
 <20181107050019.6663-3-lijiang@redhat.com>
 <20181107052345.GQ27491@MiWiFi-R3L-srv>
 <cc579d02-19e9-8749-ea48-3ff0fbf6461d@redhat.com>
 <20181109065146.GD31064@MiWiFi-R3L-srv>
From:   lijiang <lijiang@redhat.com>
Message-ID: <9cac4b46-2b45-b107-0e69-679acda170c5@redhat.com>
Date:   Fri, 9 Nov 2018 15:13:47 +0800
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在 2018年11月09日 14:51, Baoquan He 写道:
> On 11/07/18 at 05:10pm, lijiang wrote:
>> In fact, these patches are really simple. As the subject mentioned, this patch
>> [PATCH 2/2] adds the reserved e820 ranges to kdump kernel e820 table, and the
>> first patch [PATCH 1/2] helps to exactly add the e820(E820_TYPE_RESERVED) type
>> to kdump kernel e820 table, that is to say, it will filter out some unnecessary
>> type(E820_TYPE_RAM/E820_TYPE_UNUSABLE/E820_TYPE_RESERVED_KERN).
>>
>> At present, when we use the kexec to load the kernel image and initramfs(for
>> example: kexec -s -p xxxx), the latest kernel does not pass the e820 reserved
>> ranges to the second kernel, which might produce two problems:
>>
>> The first one is the MMCONFIG issue, although which does not make the system
>> crash or hang, this issue is still a potential risk, because my test can't
>> cover all cases due to resource constraints(Machine), and i'm not sure what
>> it will happen on other machine.
>>
>> The second issue is that the e820 reserved ranges do not setup in kdump kernel,
>> which will cause some functions which are related to the e820 reserved ranges
>> to become invalid. For example:
>>
>> early_memremap()->
>> early_memremap_pgprot_adjust()->
>> memremap_should_map_decrypted()->
>> e820__get_entry_type()
>>
>> Please focus on these functions, early_memremap_pgprot_adjust() and memremap_should_map_decrypted().
>>
>> In the first kernel, these ranges sit in e820 reserved ranges, so the memremap_should_map_decrypted()
>> will return true, that is to say, the reserved memory is decrypted, then the early_memremap_pgprot_adjust()
>> will call the pgprot_decrypted() to clear the memory encryption mask.
>>
>> In the second kernel, because the e820 reserved ranges are not passed to the second kernel, these ranges
>> don't sit in the e820 reserved ranges, so the the memremap_should_map_decrypted() will return false, that
>> is to say, the reserved memory is encrypted, and then the early_memremap_pgprot_adjust() will also call the
>> pgprot_encrypted() to set the memory encryption mask.
>>
>> Obviously, in the second kernel, the e820 reserved memory is still decrypted, it has gone wrong. So, if
>> don't fix, kdump won't work when we use the command(kexec -s -p xxx) to load the kernel image and initramfs.
> 
> Yes, this looks better. In fact, I searched cover-letter and both patch
> lg, didn't find anything to tell what will happen without these
> patchset. We should at least tell the patch is for cleanup/improvement/fix/feature.
> This "if don't fix, kdump won't work ..." is the sentence I first saw
> telling the sequence or the problem.
> 
> Please do not misunderstand those questions from me with unfriendly tone,
> words are cold and can't express emotions exactly, and appearance. Those
> questions are just meaning that people may get confusion from those
> places.
> 

Don't worry.
 
I would like to improve patch log based on your suggestion.

Regards,
Lianbo
> What: describe the problem you met.
> Why:  tell the root cause
> How:  how you fix it
> 
> This is what I usually use to write patch log. Just for your reference.
> 

Ok, thanks.

> Thanks
> Baoquan
>