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

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Date:   Mon, 8 May 2017 15:24:43 +0800
From:   Xunlei Pang <xpang@...hat.com>
To:     Ingo Molnar <mingo@...nel.org>, xlpang@...hat.com
Cc:     linux-kernel@...r.kernel.org, kexec@...ts.infradead.org,
        akpm@...ux-foundation.org, Eric Biederman <ebiederm@...ssion.com>,
        Dave Young <dyoung@...hat.com>, x86@...nel.org,
        Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Yinghai Lu <yinghai@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>, Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH v3 2/2] x86_64/kexec: Use PUD level 1GB page for identity
 mapping if available

On 05/08/2017 at 02:29 PM, Ingo Molnar wrote:
> * Xunlei Pang <xpang@...hat.com> wrote:
>
>> On 05/05/2017 at 05:20 PM, Ingo Molnar wrote:
>>> * Xunlei Pang <xpang@...hat.com> wrote:
>>>
>>>> On 05/05/2017 at 02:52 PM, Ingo Molnar wrote:
>>>>> * Xunlei Pang <xlpang@...hat.com> 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.
> Ok, you are right - I'll use the original patches as submitted, with the updated 
> changelogs.

Thanks!

Regards,
Xunlei