From: Quentin Casasnovas <quentin.casasnovas@oracle.com>
To: Borislav Petkov <bp@alien8.de>
Cc: Quentin Casasnovas <quentin.casasnovas@oracle.com>,
X86 ML <x86@kernel.org>, LKML <linux-kernel@vger.kernel.org>,
"H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@kernel.org>,
Thomas Gleixner <tglx@linutronix.de>,
Oleg Nesterov <oleg@redhat.com>,
Andy Lutomirski <luto@amacapital.net>
Subject: Re: [PATCH] x86/xsave: Robustify and merge macros
Date: Sat, 4 Apr 2015 12:11:55 +0200 [thread overview]
Message-ID: <20150404101155.GB5922@chrystal.uk.oracle.com> (raw)
In-Reply-To: <20150404092536.GB21152@pd.tnic>
On Sat, Apr 04, 2015 at 11:25:36AM +0200, Borislav Petkov wrote:
> On Sat, Apr 04, 2015 at 10:36:11AM +0200, Quentin Casasnovas wrote:
> > Since all of these are compile time constants, could we not use the safe
> > variant on that same page? Not that I'm too worried about the signed right
> > shift but heh that would be portable and should not impact performance
> > anyway, so no added value in using the optimized version is there?
>
> Seems to work with the experimental diff below. I need to do
>
> -(-(x < y))
>
> with the last term though as we're working with s32s.
>
> ---
> diff --git a/arch/x86/include/asm/alternative-asm.h b/arch/x86/include/asm/alternative-asm.h
> index 44a1fc5439d3..2cb6da2716bf 100644
> --- a/arch/x86/include/asm/alternative-asm.h
> +++ b/arch/x86/include/asm/alternative-asm.h
> @@ -53,14 +53,14 @@
> * Shamelessly stolen and adapted from:
> * http://graphics.stanford.edu/~seander/bithacks.html#IntegerMinOrMax
> */
> -#define alt_max_short(a,b) (((a) - (((a) - (b)) & (((a) - (b)) >> 15))) & 0xffff)
> +#define alt_max_short(a, b) ((a) ^ (((a) ^ (b)) & -(-((a) < (b)))))
>
So I'm not claiming I've spent the time to fully understand this macro but
it looks like it's doing the right thing on my dummy tests:
http://pastebin.com/DDhtZQgX
Did you also change it in the alternative.h file BTW?
> .macro ALTERNATIVE_2 oldinstr, newinstr1, feature1, newinstr2, feature2
> 140:
> \oldinstr
> 141:
> - .skip -((alt_max_short(new_len1, new_len2) - old_len) > 0) * \
> - (alt_max_short(new_len1, new_len2) - old_len),0x90
> + .skip -((alt_max_short(new_len1, new_len2) - (old_len)) > 0) * \
> + (alt_max_short(new_len1, new_len2) - (old_len)),0x90
> 142:
Good catch for the missing parenthesis!
> >
> > Looks good to me and I find it much easier to understand here :)
> >
>
> Cool. Please give it more critical staring as we're under time pressure
> here.
>
So I _think_ it's OK but it would be re-assuring if somebody else could
have a look as well just in case.. :)
Do you have a cleaned up version of the patch you're planning to apply on
top of tip/master instead of just snippets? This way we can hammer it with
different calls to ALTERNATIVE_2 and alternative_2 to check it's good?
I'll have to leave soonish though..
Quentin
next prev parent reply other threads:[~2015-04-04 10:10 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-04-02 13:11 [PATCH] x86/xsave: Robustify and merge macros Borislav Petkov
2015-04-02 15:52 ` Quentin Casasnovas
2015-04-02 16:12 ` Borislav Petkov
2015-04-02 16:33 ` Quentin Casasnovas
2015-04-02 16:45 ` Borislav Petkov
2015-04-03 14:06 ` Quentin Casasnovas
2015-04-03 14:14 ` Quentin Casasnovas
2015-04-03 15:23 ` Borislav Petkov
2015-04-03 15:40 ` Quentin Casasnovas
2015-04-03 17:06 ` Borislav Petkov
2015-04-03 17:33 ` Quentin Casasnovas
2015-04-03 17:48 ` Borislav Petkov
2015-04-03 20:42 ` Quentin Casasnovas
2015-04-04 7:34 ` Borislav Petkov
2015-04-04 8:36 ` Quentin Casasnovas
2015-04-04 9:25 ` Borislav Petkov
2015-04-04 10:11 ` Quentin Casasnovas [this message]
2015-04-04 10:29 ` Borislav Petkov
2015-04-04 13:32 ` Borislav Petkov
2015-04-04 13:34 ` [PATCH] x86/alternatives: Fix ALTERNATIVE_2 padding generation properly Borislav Petkov
2015-04-07 9:27 ` Quentin Casasnovas
2015-04-07 9:40 ` [tip:x86/asm] " tip-bot for Borislav Petkov
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