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Subject: Re: [PATCH] x86/kdump: Fix 'kmem -s' reported an invalid freepointer
 when SME was active
To:     Baoquan He <bhe@redhat.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     Dave Young <dyoung@redhat.com>, linux-kernel@vger.kernel.org,
        tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, hpa@zytor.com,
        x86@kernel.org, jgross@suse.com, dhowells@redhat.com,
        Thomas.Lendacky@amd.com, kexec@lists.infradead.org,
        Vivek Goyal <vgoyal@redhat.com>
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From:   lijiang <lijiang@redhat.com>
Message-ID: <7e97421d-d9a8-9d57-1aa0-406039f8421d@redhat.com>
Date:   Sat, 5 Oct 2019 15:35:07 +0800
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在 2019年10月01日 15:40, Baoquan He 写道:
> On 09/30/19 at 05:14am, Eric W. Biederman wrote:
>> Baoquan He <bhe@redhat.com> writes:
>>>> needs a little better description.  I know it is not a lot on modern
>>>> systems but reserving an extra 1M of memory to avoid having to special
>>>> case it later seems in need of calling out.
>>>>
>>>> I have an old system around that I think that 640K is about 25% of
>>>> memory.
>>>
>>> Understood. Basically 640K is wasted in this case. But we only do like
>>> this in SME case, a condition checking is added. And system with SME is
>>> pretty new model, it may not impact the old system.
>>
>> The conditional really should be based on if we are reserving memory
>> for a kdump kernel.  AKA if crash_kernel=XXX is specified on the kernel
>> command line.
>>
>> At which point I think it would be very reasonable to unconditionally
>> reserve the low 640k, and make the whole thing a non-issue.  This would
>> allow the kdump code to just not do anything special for any of the
>> weird special case.
>>
>> It isn't perfect because we need a page or so used in the first kernel
>> for bootstrapping the secondary cpus, but that seems like the least of
>> evils.  Especially as no one will DMA to that memory.
>>
>> So please let's just change what memory we reserve when crash_kernel is
>> specified.
> 
> Yes, makes sense, thanks for pointing it out.
> 

Sorry for the delay and thanks for your comment, Eric, Baoquan and Dave Young.

I will improve patch log and add the extra condition crash_kernel. I will post
v2 later.

Thanks.
Lianbo

>>
>>>> How we interact with BIOS tables in the first 640k needs some
>>>> explanation.  Both in the first kernel and in the crash kernel.
>>>
>>> Yes, totally agree.
>>>
>>> Those BIOS tables have been reserved as e820 reserved regions and will
>>> be passed to kdump kernel for reusing. Memblock reserved 640K doesn't
>>> mean it will cover the whole [0, 640K) region, it only searches for
>>> available system RAM from memblock allocator.
>>
>> Careful with that assumption.  My memory is that the e820 memory map
>> frequently fails to cover areas like the real mode interrupt descriptor
>> table at address 0.
> 
> OK, will think more about this. Thanks.
>