Re: [PATCH v3 2/2] x86_64/kexec: Use PUD level 1GB page for identity mapping if available

[Xunlei Pang]

On 05/05/2017 at 05:20 PM, Ingo Molnar wrote:
> * Xunlei Pang <xpang@xxxxxxxxxx> wrote:
>
>> On 05/05/2017 at 02:52 PM, Ingo Molnar wrote:
>>> * Xunlei Pang <xlpang@xxxxxxxxxx> wrote:
>>>
>>>> @@ -122,6 +122,10 @@ static int init_pgtable(struct kimage *image, unsigned long start_pgtable)
>>>>  
>>>>  	level4p = (pgd_t *)__va(start_pgtable);
>>>>  	clear_page(level4p);
>>>> +
>>>> +	if (direct_gbpages)
>>>> +		info.direct_gbpages = true;
>>> No, this should be keyed off the CPU feature (X86_FEATURE_GBPAGES) automatically, 
>>> not set blindly! AFAICS this patch will crash kexec on any CPU that does not 
>>> support gbpages.
>> It should be fine, probe_page_size_mask() already takes care of this:
>>     if (direct_gbpages && boot_cpu_has(X86_FEATURE_GBPAGES)) {
>>         printk(KERN_INFO "Using GB pages for direct mapping\n");
>>         page_size_mask |= 1 << PG_LEVEL_1G;
>>     } else {
>>         direct_gbpages = 0;
>>     }
>>
>> So if X86_FEATURE_GBPAGES is not supported, direct_gbpages will be set to 0.
> So why is the introduction of the info.direct_gbpages flag necessary? AFAICS it 
> just duplicates the kernel's direct_gbpages flag. One outcome is that hibernation 
> won't use gbpages, which is silly.

boot/compressed/pagetable.c  also uses kernel_ident_mapping_init() for kaslr, at the moment
we don't have "direct_gbpages" definition or X86_FEATURE_GBPAGES feature detection.

I thought that we can change the other call sites when found really needed.

Regards,
Xunlei