Date:   Wed, 16 Oct 2019 16:40:29 +0800
From:   lijiang <lijiang@...hat.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Dave Young <dyoung@...hat.com>, linux-kernel@...r.kernel.org,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, hpa@...or.com,
        x86@...nel.org, bhe@...hat.com, jgross@...e.com,
        dhowells@...hat.com, Thomas.Lendacky@....com, vgoyal@...hat.com,
        kexec@...ts.infradead.org
Subject: Re: [PATCH 3/3 v3] x86/kdump: clean up all the code related to the
 backup region

在 2019年10月15日 19:04, Eric W. Biederman 写道:
> lijiang <lijiang@...hat.com> writes:
> 
>> 在 2019年10月13日 11:54, Eric W. Biederman 写道:
>>> Dave Young <dyoung@...hat.com> writes:
>>>
>>>> Hi Eric,
>>>>
>>>> On 10/12/19 at 06:26am, Eric W. Biederman wrote:
>>>>> Lianbo Jiang <lijiang@...hat.com> writes:
>>>>>
>>>>>> When the crashkernel kernel command line option is specified, the
>>>>>> low 1MiB memory will always be reserved, which makes that the memory
>>>>>> allocated later won't fall into the low 1MiB area, thereby, it's not
>>>>>> necessary to create a backup region and also no need to copy the first
>>>>>> 640k content to a backup region.
>>>>>>
>>>>>> Currently, the code related to the backup region can be safely removed,
>>>>>> so lets clean up.
>>>>>>
>>>>>> Signed-off-by: Lianbo Jiang <lijiang@...hat.com>
>>>>>> ---
>>>>>
>>>>>> diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c
>>>>>> index eb651fbde92a..cc5774fc84c0 100644
>>>>>> --- a/arch/x86/kernel/crash.c
>>>>>> +++ b/arch/x86/kernel/crash.c
>>>>>> @@ -173,8 +173,6 @@ void native_machine_crash_shutdown(struct pt_regs *regs)
>>>>>>  
>>>>>>  #ifdef CONFIG_KEXEC_FILE
>>>>>>  
>>>>>> -static unsigned long crash_zero_bytes;
>>>>>> -
>>>>>>  static int get_nr_ram_ranges_callback(struct resource *res, void *arg)
>>>>>>  {
>>>>>>  	unsigned int *nr_ranges = arg;
>>>>>> @@ -234,9 +232,15 @@ static int prepare_elf64_ram_headers_callback(struct resource *res, void *arg)
>>>>>>  {
>>>>>>  	struct crash_mem *cmem = arg;
>>>>>>  
>>>>>> -	cmem->ranges[cmem->nr_ranges].start = res->start;
>>>>>> -	cmem->ranges[cmem->nr_ranges].end = res->end;
>>>>>> -	cmem->nr_ranges++;
>>>>>> +	if (res->start >= SZ_1M) {
>>>>>> +		cmem->ranges[cmem->nr_ranges].start = res->start;
>>>>>> +		cmem->ranges[cmem->nr_ranges].end = res->end;
>>>>>> +		cmem->nr_ranges++;
>>>>>> +	} else if (res->end > SZ_1M) {
>>>>>> +		cmem->ranges[cmem->nr_ranges].start = SZ_1M;
>>>>>> +		cmem->ranges[cmem->nr_ranges].end = res->end;
>>>>>> +		cmem->nr_ranges++;
>>>>>> +	}
>>>>>
>>>>> What is going on with this chunk?  I can guess but this needs a clear
>>>>> comment.
>>>>
>>>> Indeed it needs some code comment, this is based on some offline
>>>> discussion.  cat /proc/vmcore will give a warning because ioremap is
>>>> mapping the system ram.
>>>>
>>>> We pass the first 1M to kdump kernel in e820 as system ram so that 2nd
>>>> kernel can use the low 1M memory because for example the trampoline
>>>> code.
>>>>
>>>>>
>>>>>>  
>>>>>>  	return 0;
>>>>>>  }
>>>>>
>>>>>> @@ -356,9 +337,12 @@ int crash_setup_memmap_entries(struct kimage *image, struct boot_params *params)
>>>>>>  	memset(&cmd, 0, sizeof(struct crash_memmap_data));
>>>>>>  	cmd.params = params;
>>>>>>  
>>>>>> -	/* Add first 640K segment */
>>>>>> -	ei.addr = image->arch.backup_src_start;
>>>>>> -	ei.size = image->arch.backup_src_sz;
>>>>>> +	/*
>>>>>> +	 * Add the low memory range[0x1000, SZ_1M], skip
>>>>>> +	 * the first zero page.
>>>>>> +	 */
>>>>>> +	ei.addr = PAGE_SIZE;
>>>>>> +	ei.size = SZ_1M - PAGE_SIZE;
>>>>>>  	ei.type = E820_TYPE_RAM;
>>>>>>  	add_e820_entry(params, &ei);
>>>>>
>>>>> Likewise here.  Why do we need a special case?
>>>>> Why the magic with PAGE_SIZE?
>>>>
>>>> Good catch, the zero page part is useless, I think no other special
>>>> reason, just assumed zero page is not usable, but it should be ok to
>>>> remove the special handling, just pass 0 - 1M is good enough.
>>>
>>> But if we have stopped special casing the low 1M.  Why do we need a
>>> special case here at all?
>>>
>> Here, need to pass the low memory range to kdump kernel, which will guarantee
>> the availability of low memory in kdump kernel, otherwise, kdump kernel won't
>> use the low memory region.
>>
>>> If you need the special case it is almost certainly wrong to say you
>>> have ram above 640KiB and below 1MiB.  That is the legacy ROM and video
>>> MMIO area.
>>>
>>> There is a reason the original code said 640KiB.
>>>
>> Do you mean that the 640k region is good enough here instead of 1MiB?
> 
> Reading through the code of crash_setup_memap_entries I see that what
> the code is doing now.  The code is repeating the e820 memory map with
> the memory areas that were not reserved for the crash kernel removed.
> 
> In which case what the code needs to be doing something like:
> 
> 	cmd.type = E820_TYPE_RAM;
> 	flags = IORESOURCE_MEM;
> 	walk_iomem_res_desc(IORES_DESC_RESERVED, flags, 0, 1024*1024, &cmd,
> 			       memmap_entry_callback);
> 
> Depending on which bugs exist it might make sense to limit this to
> the low 640KiB.  But finding something the kernel already recognizes
> as RAM should prevent most of those problems already.  Barring bugs
> I admit it doesn't make sense to repeat the work that someone else
> has already done.
> 
> This bit:
> 	/* Add e820 reserved ranges */
> 	cmd.type = E820_TYPE_RESERVED;
> 	flags = IORESOURCE_MEM;
> 	walk_iomem_res_desc(IORES_DESC_RESERVED, flags, 0, -1, &cmd,
> 			   memmap_entry_callback);
> 
> Should probably start at 1MiB instead of 0.  Just so we don't report the
> memory below 1MiB as unconditionally reserved.   I don't properly
> understand the IORES_DESC_RESERVED flag, and how that differs from
> flags.  So please test my suggestions to verify the code works as
> expected.
> 
Thanks for your comment, Eric.

I will make a test based on your suggestions. But i need an SME machine,
maybe i will reply later.

Thanks.
Lianbo

> Eric
> 

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