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

[Ingo Molnar <mingo@kernel.org>]

* 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.

Thanks,

	Ingo