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Date:      Thu, 15 Nov 2001 14:12:32 -0800 (PST)
From:      John Baldwin <jhb@FreeBSD.org>
To:        Maxim Sobolev <sobomax@FreeBSD.org>
Cc:        hackers@FreeBSD.org, freebsd-ports@FreeBSD.org, marcus@marcuscom.com
Subject:   RE: Using bit 21 of EFLAGS in user-mode [was: Re: sigreturn: efl
Message-ID:  <XFMail.011115141232.jhb@FreeBSD.org>
In-Reply-To: <200111152156.XAA79972@ipcard.iptcom.net>

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On 15-Nov-01 Maxim Sobolev wrote:
> That explains... After a quick glance at png code I found that
> the only place where EFLAGS is altered is CPUID code, where
> the library flips bit 21 of EFLAGS in order to ensure that the
> CPUID instruction is supported (otherwise it will get SIGILL
> on older processors). Unfortunately, for some reason FreeBSB
> kernel considers bit 21 of EFLAGS as one that should not be
> altered in the user mode, thus making it illegal to use standard
> user-mode processor-detection routines based around that bit.
> AFAIK, it is a bug in FreeBSD, because there is actually nothing
> wrong with altering bit 21 in the user mode - it doesn't have
> any side effects and pretty much any of the currently available
> on the i386 OSes allows it.

Agreed, patch looks good to me.

-- 

John Baldwin <jhb@FreeBSD.org>  <><  http://www.FreeBSD.org/~jhb/
"Power Users Use the Power to Serve!"  -  http://www.FreeBSD.org/

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