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Date: Mon, 8 May 2017 08:29:32 +0200
From: Ingo Molnar <mingo@kernel.org>
Subject: Re: [PATCH v3 2/2] x86_64/kexec: Use PUD level 1GB page for identity
 mapping if available
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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>


* Xunlei Pang <xpang@redhat.com> wrote:

> 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.

Ok, you are right - I'll use the original patches as submitted, with the updated 
changelogs.

Thanks,

	Ingo

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