linux-kernel - Re: [tip:x86/mm] x86/asm: Remove __VIRTUAL_MASK

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Message-ID: <20170405120029.ckl2a5bc3ikydozw@black.fi.intel.com>
Date:   Wed, 5 Apr 2017 15:00:30 +0300
From:   "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:     Denys Vlasenko <dvlasenk@...hat.com>
Cc:     brgerst@...il.com, torvalds@...ux-foundation.org,
        akpm@...ux-foundation.org, luto@...capital.net,
        linux-kernel@...r.kernel.org, jpoimboe@...hat.com, luto@...nel.org,
        peterz@...radead.org, bp@...en8.de, hpa@...or.com,
        dave.hansen@...el.com, tglx@...utronix.de, mingo@...nel.org,
        linux-tip-commits@...r.kernel.org
Subject: Re: [tip:x86/mm] x86/asm: Remove __VIRTUAL_MASK_SHIFT==47 assert

On Wed, Apr 05, 2017 at 01:50:27PM +0200, Denys Vlasenko wrote:
> Er.... "Change top bits ... ((47 or 56 [bits] depending on paging mode)"?
> I know that's wrong and that's not what you meant to say,
> but it can be read this way too. "47th" instead of "47"
> would eliminate this reading, but you removed "th".
> 
> Spell it out to eliminate any chance of confusion:
> 
> 	Change top bits to match most significant bit (47th or 56th bit
> 	depending on paging mode) in the address.
> 

---------------------8<-----------------------

>From 5dec28ba49387ff444e447213b553506e253616d Mon Sep 17 00:00:00 2001
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Date: Wed, 5 Apr 2017 14:06:15 +0300
Subject: [PATCH] x86/asm: Fix comment in return_from_SYSCALL_64

On x86-64 __VIRTUAL_MASK_SHIFT depends on paging mode now.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
---
 arch/x86/entry/entry_64.S | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 607d72c4a485..edec30584eb8 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -266,7 +266,8 @@ return_from_SYSCALL_64:
 	 * If width of "canonical tail" ever becomes variable, this will need
 	 * to be updated to remain correct on both old and new CPUs.
 	 *
-	 * Change top 16 bits to be the sign-extension of 47th bit
+	 * Change top bits to match most significant bit (47th or 56th bit
+	 * depending on paging mode) in the address.
 	 */
 	shl	$(64 - (__VIRTUAL_MASK_SHIFT+1)), %rcx
 	sar	$(64 - (__VIRTUAL_MASK_SHIFT+1)), %rcx
-- 
 Kirill A. Shutemov