From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:50632) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1d50br-0004nj-37 for qemu-devel@nongnu.org; Sun, 30 Apr 2017 21:58:44 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1d50bo-0006V1-28 for qemu-devel@nongnu.org; Sun, 30 Apr 2017 21:58:43 -0400 Received: from resqmta-po-11v.sys.comcast.net ([96.114.154.170]:53670) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1d50bn-0006Ul-Sg for qemu-devel@nongnu.org; Sun, 30 Apr 2017 21:58:39 -0400 From: Michael Eager Message-ID: <5906960F.2050305@eagerm.com> Date: Sun, 30 Apr 2017 18:57:35 -0700 MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Subject: [Qemu-devel] cpu_io_recompile, icount, and re-issued instructions List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: qemu-devel I'm working with an emulation for a proprietary processor on an older QEMU source base. It looks like the problem I am seeing in the old sources would still be present in the current source base. I'm seeing incorrect values when there is a write to a memory-mapped I/O device when icount is set. What I see happening is that a TB with ~20 instructions is executed which contains a write to the MM I/O address. When it gets to the io_write routine, can_do_io is false, which results in a call to cpu_io_recompile. cpu_io_recompile does what it (sort of) says it is supposed to do: it builds a new TB with the I/O instruction as the last instruction in the block, then re-issues the TB. The problem is that the new TB contains the instructions before the I/O instruction, so they are executed a second time. There is a note in cpu_io_recompile which mentions the situation where the I/O instruction is not the first in the TB has not been handled: /* TODO: If env->pc != tb->pc (i.e. the faulting instruction was not * the first in the TB) then we end up generating a whole new TB and * repeating the fault, which is horribly inefficient. * Better would be to execute just this insn uncached, or generate a * second new TB. I'm a bit unclear what this is saying. A new TB is generated which is issued setting can_do_io, and which passes through io_write without a problem. I'm not sure what fault is repeated. It seems to me that what cpu_io_recompile should do is create a new TB with the n instructions which were already executed and cache it. Then it should create a TB with only the I/O instruction, setting can_do_io, and re-issue this TB. Am I understanding this correctly? This code has been around for a long, long time. Has anyone noticed this problem in the past? -- Michael Eager eager@eagercon.com 1960 Park Blvd., Palo Alto, CA 94306 650-325-8077 -- Michael Eager eager@eagercon.com 1960 Park Blvd., Palo Alto, CA 94306 650-325-8077