From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:60952) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1d5SiI-00010a-Cn for qemu-devel@nongnu.org; Tue, 02 May 2017 03:59:15 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1d5SiF-0004JV-CE for qemu-devel@nongnu.org; Tue, 02 May 2017 03:59:14 -0400 Received: from mx1.redhat.com ([209.132.183.28]:44658) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1d5SiF-0004J2-6H for qemu-devel@nongnu.org; Tue, 02 May 2017 03:59:11 -0400 References: <5906960F.2050305@eagerm.com> From: Paolo Bonzini Message-ID: <3a7aa06d-a419-69d7-a54a-e963d4ba748f@redhat.com> Date: Tue, 2 May 2017 09:59:03 +0200 MIME-Version: 1.0 In-Reply-To: <5906960F.2050305@eagerm.com> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Subject: Re: [Qemu-devel] cpu_io_recompile, icount, and re-issued instructions List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Michael Eager , qemu-devel On 01/05/2017 03:57, Michael Eager wrote: >=20 >=20 > 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.=20 > When it gets to the io_write routine, can_do_io is false, which results > in a call to cpu_io_recompile. >=20 > 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. They shouldn't. When called from cpu_io_recompile, cpu_restore_state_from_tb should compute the I/O instruction's target PC from the host PC (stored in retaddr). Then what happens is the following: - cpu_io_recompile generates a new TB ending with the I/O instruction. This new TB has a hash table conflict with the old TB (same PC/cs_base/flags) the old TB is implicitly removed - cpu_io_recompile calls cpu_loop_exit_noexc, which goes back to the execution loop with updated PC - because the PC is different, a new TB is looked up for the I/O instruction's PC. The TB probably is not there and translation starts again, this time at the I/O instruction - the new TB, when executed, causes cpu_io_recompile to fire again. This is the inefficient part mentioned in cpu_io_recompile - cpu_io_recompile now compiles a one-instruction TB and goes back to the execution loop - finally the execution loop executes the one-instruction TB for the I/O instruction, then it can go on Thanks, Paolo