From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:47503) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1WkKd7-0005VE-Tk for qemu-devel@nongnu.org; Tue, 13 May 2014 17:53:03 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1WkKd0-000067-OW for qemu-devel@nongnu.org; Tue, 13 May 2014 17:52:57 -0400 Received: from mail-qg0-x22d.google.com ([2607:f8b0:400d:c04::22d]:43001) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1WkKd0-000062-Ju for qemu-devel@nongnu.org; Tue, 13 May 2014 17:52:50 -0400 Received: by mail-qg0-f45.google.com with SMTP id z60so1415686qgd.32 for ; Tue, 13 May 2014 14:52:50 -0700 (PDT) Sender: Richard Henderson Message-ID: <5372942E.2060901@twiddle.net> Date: Tue, 13 May 2014 14:52:46 -0700 From: Richard Henderson MIME-Version: 1.0 References: <1400016125-4869-1-git-send-email-rth@twiddle.net> In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH 0/3] tci rewrite List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Peter Maydell Cc: Stefan Weil , QEMU Developers On 05/13/2014 02:49 PM, Peter Maydell wrote: > On 13 May 2014 22:22, Richard Henderson wrote: >> There are a number of problems with the current interpreter: >> >> (1) It uses direct unaligned accesses as part of the bytecode stream. >> This fails with SIGBUS for strict alignment hosts. This could of >> course be fixed to use "proper" unaligned accesses, but this would >> just be slow. >> >> (2) The method by which it implements calls is non-portable, and really >> only works for x86. >> >> (3) The code is full of ifdefs and TODOs that never got completed. >> >> (4) The "registers" are in a global "tci_reg" array that implies that >> if threads are ever used, we'll immediately get corruption. Fixing >> this is complicated by the structure of the interpreter. >> >> (5) It hasn't been updated to the "new" ldst opcodes. >> >> To me, all of this adds up to a complete rewrite. > > Does the TCG interpreter actually have any interesting uses > to merit our carrying it around, maintaining it and (in this case) > rewriting it? Mostly I just see it when people accidentally configure > it and I have to say "don't do that, you really want the native TCG > backend". The only interesting use I can think of is pure portability to otherwise unsupported hosts. Rare these days, but there are still some kicking. When do folks accidentally configure it? r~