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 for kexec@lists.infradead.org; Fri, 05 May 2017 10:50:09 +0000
Subject: Re: [PATCH v3 2/2] x86_64/kexec: Use PUD level 1GB page for identity
 mapping if available
References: <1493862171-8799-1-git-send-email-xlpang@redhat.com>
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From: Xunlei Pang <xpang@redhat.com>
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Date: Fri, 5 May 2017 18:50:58 +0800
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To: Ingo Molnar <mingo@kernel.org>, xlpang@redhat.com
Cc: Yinghai Lu <yinghai@kernel.org>, x86@kernel.org, kexec@lists.infradead.org, linux-kernel@vger.kernel.org, Borislav Petkov <bp@alien8.de>, Eric Biederman <ebiederm@xmission.com>, Andy Lutomirski <luto@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>, akpm@linux-foundation.org, Dave Young <dyoung@redhat.com>, Thomas Gleixner <tglx@linutronix.de>

On 05/05/2017 at 05:20 PM, Ingo Molnar wrote:
> * Xunlei Pang <xpang@redhat.com> wrote:
>
>> On 05/05/2017 at 02:52 PM, Ingo Molnar wrote:
>>> * Xunlei Pang <xlpang@redhat.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.

Regards,
Xunlei


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