Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756870AbbFPLQu (ORCPT ); Tue, 16 Jun 2015 07:16:50 -0400 Received: from mail.skyhub.de ([78.46.96.112]:39181 "EHLO mail.skyhub.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752270AbbFPLQl (ORCPT ); Tue, 16 Jun 2015 07:16:41 -0400 Date: Tue, 16 Jun 2015 13:16:32 +0200 From: Borislav Petkov To: Alexander Popov Cc: Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , Andrey Konovalov , Andrew Morton , Andrey Ryabinin , Andy Lutomirski , Alexander Kuleshov , Denys Vlasenko , Peter Zijlstra , Kees Cook , x86@kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v5 1/2] x86_64: remove not needed clear_page for init_level4_page Message-ID: <20150616111632.GF17786@pd.tnic> References: <1433842921-18055-1-git-send-email-alpopov@ptsecurity.com> <1433842921-18055-2-git-send-email-alpopov@ptsecurity.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <1433842921-18055-2-git-send-email-alpopov@ptsecurity.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3365 Lines: 100 On Tue, Jun 09, 2015 at 12:42:00PM +0300, Alexander Popov wrote: > From: Andrey Ryabinin > > Commit 8170e6bed465 ("x86, 64bit: Use a #PF handler to materialize > early mappings on demand") introduced clear_page(init_level4_pgt); > call in x86_64_start_kernel(). However, this clear_page is useless > because init_level4_page already filled with zeroes in head_64.S I really doubt that, see below: > Commit message in 8170e6bed465 says that this clear_page() was > dropped in v7, but it accidentally reappeared in later versions > of that patchset. > > Signed-off-by: Andrey Ryabinin > --- > arch/x86/kernel/head64.c | 1 - > 1 file changed, 1 deletion(-) > > diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c > index 5a46681..6a6eefd 100644 > --- a/arch/x86/kernel/head64.c > +++ b/arch/x86/kernel/head64.c > @@ -177,7 +177,6 @@ asmlinkage __visible void __init x86_64_start_kernel(char * real_mode_data) > */ > load_ucode_bsp(); > > - clear_page(init_level4_pgt); > /* set init_level4_pgt kernel high mapping*/ > init_level4_pgt[511] = early_level4_pgt[511]; vmlinux has: ffffffff81a00000 : ffffffff81a00000: 63 10 movslq (%rax),%edx ffffffff81a00002: a0 01 00 00 00 00 00 movabs 0x1,%al ffffffff81a00009: 00 00 ... ffffffff81a0087f: 00 63 10 add %ah,0x10(%rbx) ffffffff81a00882: a0 01 00 00 00 00 00 movabs 0x1,%al ffffffff81a00889: 00 00 ... ffffffff81a00ff7: 00 67 30 add %ah,0x30(%rdi) ffffffff81a00ffa: a0 01 00 00 00 00 63 movabs 0xa020630000000001,%al ffffffff81a01001: 20 a0 ffffffff81a01000 : ffffffff81a01000: 63 20 movslq (%rax),%esp ffffffff81a01002: a0 01 00 00 00 00 00 movabs 0x1,%al ffffffff81a01009: 00 00 ... Booting a kvm quest and halting it before the clear_page: --- /* * Load microcode early on BSP. */ load_ucode_bsp(); // clear_page(init_level4_pgt); while(1) cpu_relax(); /* set init_level4_pgt kernel high mapping*/ init_level4_pgt[511] = early_level4_pgt[511]; --- and dumping the memory at ffffffff81a00000 gives: --- 00000000 63 10 a0 01 00 00 00 00 00 00 00 00 00 00 00 00 |c...............| 00000010 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| * 00000880 63 10 a0 01 00 00 00 00 00 00 00 00 00 00 00 00 |c...............| 00000890 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| * 00000ff0 00 00 00 00 00 00 00 00 67 30 a0 01 00 00 00 00 |........g0......| 00001000 --- These are basically the PTE entries it gets for the CONFIG_XEN case which we clear because we're on baremetal. Now my hunch is that those entries get overwritten later but I wouldn't want to debug any strange bugs from leftovers in init_level4_pgt so having the clear_page() is actually a good thing. -- Regards/Gruss, Boris. ECO tip #101: Trim your mails when you reply. -- -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/