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Date: Fri, 5 May 2017 18:50:58 +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 05/05/2017 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. Regards, Xunlei
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