From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from [140.186.70.92] (port=36176 helo=eggs.gnu.org)
	by lists.gnu.org with esmtp (Exim 4.43) id 1OUiw1-0004in-0A
	for qemu-devel@nongnu.org; Fri, 02 Jul 2010 12:17:50 -0400
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.69)
	(envelope-from <rth@twiddle.net>) id 1OUivz-00077d-L2
	for qemu-devel@nongnu.org; Fri, 02 Jul 2010 12:17:48 -0400
Received: from b.mail.sonic.net ([64.142.19.5]:33694)
	by eggs.gnu.org with esmtp (Exim 4.69)
	(envelope-from <rth@twiddle.net>) id 1OUivz-00077V-Ch
	for qemu-devel@nongnu.org; Fri, 02 Jul 2010 12:17:47 -0400
Message-ID: <4C2E1105.2080903@twiddle.net>
Date: Fri, 02 Jul 2010 09:17:09 -0700
From: Richard Henderson <rth@twiddle.net>
MIME-Version: 1.0
References: <1269890225-13639-1-git-send-email-weil@mail.berlios.de>	<1269890225-13639-2-git-send-email-weil@mail.berlios.de>	<20100408192902.GI6056@volta.aurel32.net>	<4BBF0D7B.60400@mail.berlios.de>
	<4C2E07B6.1050605@redhat.com>
In-Reply-To: <4C2E07B6.1050605@redhat.com>
Content-Type: text/plain; charset=ISO-8859-15
Content-Transfer-Encoding: 7bit
Subject: [Qemu-devel] [RFC] env stored in segment register for i386
List-Id: qemu-devel.nongnu.org
List-Unsubscribe: <http://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=unsubscribe>
List-Archive: <http://lists.nongnu.org/archive/html/qemu-devel>
List-Post: <mailto:qemu-devel@nongnu.org>
List-Help: <mailto:qemu-devel-request@nongnu.org?subject=help>
List-Subscribe: <http://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=subscribe>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: QEMU Developers <qemu-devel@nongnu.org>, Aurelien Jarno <aurelien@aurel32.net>

On 07/02/2010 08:37 AM, Paolo Bonzini wrote:
> The second (more real) reason is inline assembly failures, for example
> (32-bit x86):
> 
>     register int e asm("edi");
> 
>     static inline int h()
>     {
>         int x;
>         asm volatile ("mov $0, %0" : "=D" (x));
>     }
> 
>     int g()
>     {
>         int f = e;
>         h();
>         return e - f;
>     }
> 
> fails to compile because gcc cannot assign edi to %0 in h().  Some host
> headers may use assembly in a way that breaks qemu.  With only one
> global register in use, however, it makes sense IMO to drop the custom
> inclusion hacks and see if anyone screams.

A few months ago I developed a patch that would allow the global env
variable to be accessed via %fs (plus a backing TLS variable), which
means that no hardware register needs to be reserved for i386.

I never quite got around to finishing it because I don't know how to
set up a segment register in Windows, and it seemed like the kind of
patch that could easily get quagmired.

Is there any interest in a patch like this?  Should I try to revive it?


r~