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:   Fri, 5 May 2017 11:20:46 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     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


* 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.

Thanks,

	Ingo