linux-kernel - RE: [PATCH] x86/mce: Fix set_mce

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Date:   Mon, 10 Sep 2018 13:52:24 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Linus Torvalds' <torvalds@...ux-foundation.org>,
        Tony Luck <tony.luck@...el.com>
CC:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Peter Anvin <hpa@...or.com>,
        Borislav Petkov <bp@...en8.de>,
        linux-edac <linux-edac@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Jiang <dave.jiang@...el.com>
Subject: RE: [PATCH] x86/mce: Fix set_mce_nospec() to avoid #GP fault

From: Linus Torvalds
> ...
> You could literally do something like
> 
>     /* Make it canonical in case we flipped the high bit */
>     addr = (long)(addr<<1)>>1;

Isn't it safer to use a mask and let the compiler decide if two
shifts are a good implementation?

	addr &= ~HIGH_MAGIC_BIT;

ISTR fixing a bug in gld where it had (foo << 16) >> 16 instead
of foo & 0xffff. Didn't work well on 64bit.

(I may need to recompile that old gcc/gld again.
Failed miserably last time I tried.)

	David

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