From patchwork Mon Sep 4 09:06:10 2023 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Patchwork-Submitter: =?utf-8?q?C=C3=A9dric_Le_Goater?= X-Patchwork-Id: 13373635 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id ADE2EC83F2C for ; Mon, 4 Sep 2023 09:11:43 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1qd5YH-00025k-SV; Mon, 04 Sep 2023 05:07:21 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1qd5YG-00023W-K2; Mon, 04 Sep 2023 05:07:20 -0400 Received: from gandalf.ozlabs.org ([150.107.74.76]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1qd5YD-0003ku-WE; Mon, 04 Sep 2023 05:07:20 -0400 Received: from gandalf.ozlabs.org (mail.ozlabs.org [IPv6:2404:9400:2221:ea00::3]) by gandalf.ozlabs.org (Postfix) with ESMTP id 4RfN7R53lHz4wy8; Mon, 4 Sep 2023 19:07:15 +1000 (AEST) Received: from authenticated.ozlabs.org (localhost [127.0.0.1]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (No client certificate requested) by mail.ozlabs.org (Postfix) with ESMTPSA id 4RfN7P6GdKz4wy1; Mon, 4 Sep 2023 19:07:13 +1000 (AEST) From: =?utf-8?q?C=C3=A9dric_Le_Goater?= To: qemu-ppc@nongnu.org, qemu-devel@nongnu.org Cc: Daniel Henrique Barboza , Nicholas Piggin , =?utf-8?q?C=C3=A9dric_Le_Goater?= Subject: [PULL 15/35] hw/ppc: Avoid decrementer rounding errors Date: Mon, 4 Sep 2023 11:06:10 +0200 Message-ID: <20230904090630.725952-16-clg@kaod.org> X-Mailer: git-send-email 2.41.0 In-Reply-To: <20230904090630.725952-1-clg@kaod.org> References: <20230904090630.725952-1-clg@kaod.org> MIME-Version: 1.0 Received-SPF: pass client-ip=150.107.74.76; envelope-from=SRS0=kZzc=EU=kaod.org=clg@ozlabs.org; helo=gandalf.ozlabs.org X-Spam_score_int: -16 X-Spam_score: -1.7 X-Spam_bar: - X-Spam_report: (-1.7 / 5.0 requ) BAYES_00=-1.9, HEADER_FROM_DIFFERENT_DOMAINS=0.249, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=no autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org From: Nicholas Piggin The decrementer register contains a relative time in timebase units. When writing to DECR this is converted and stored as an absolute value in nanosecond units, reading DECR converts back to relative timebase. The tb<->ns conversion of the relative part can cause rounding such that a value writen to the decrementer can read back a different, with time held constant. This is a particular problem for a deterministic icount and record-replay trace. Fix this by storing the absolute value in timebase units rather than nanoseconds. The math before: store: decr_next = now_ns + decr * ns_per_sec / tb_per_sec load: decr = (decr_next - now_ns) * tb_per_sec / ns_per_sec load(store): decr = decr * ns_per_sec / tb_per_sec * tb_per_sec / ns_per_sec After: store: decr_next = now_ns * tb_per_sec / ns_per_sec + decr load: decr = decr_next - now_ns * tb_per_sec / ns_per_sec load(store): decr = decr Fixes: 9fddaa0c0cab ("PowerPC merge: real time TB and decrementer - faster and simpler exception handling (Jocelyn Mayer)") Signed-off-by: Nicholas Piggin Signed-off-by: Cédric Le Goater --- hw/ppc/ppc.c | 39 ++++++++++++++++++++++++--------------- 1 file changed, 24 insertions(+), 15 deletions(-) diff --git a/hw/ppc/ppc.c b/hw/ppc/ppc.c index 13eb45f4b77e..a397820d9c34 100644 --- a/hw/ppc/ppc.c +++ b/hw/ppc/ppc.c @@ -711,16 +711,17 @@ bool ppc_decr_clear_on_delivery(CPUPPCState *env) static inline int64_t _cpu_ppc_load_decr(CPUPPCState *env, uint64_t next) { ppc_tb_t *tb_env = env->tb_env; - int64_t decr, diff; + uint64_t now, n; + int64_t decr; - diff = next - qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL); - if (diff >= 0) { - decr = ns_to_tb(tb_env->decr_freq, diff); - } else if (tb_env->flags & PPC_TIMER_BOOKE) { + now = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL); + n = ns_to_tb(tb_env->decr_freq, now); + if (next > n && tb_env->flags & PPC_TIMER_BOOKE) { decr = 0; - } else { - decr = -ns_to_tb(tb_env->decr_freq, -diff); + } else { + decr = next - n; } + trace_ppc_decr_load(decr); return decr; @@ -857,13 +858,18 @@ static void __cpu_ppc_store_decr(PowerPCCPU *cpu, uint64_t *nextp, (*lower_excp)(cpu); } - /* Calculate the next timer event */ + /* + * Calculate the next decrementer event and set a timer. + * decr_next is in timebase units to keep rounding simple. Note it is + * not adjusted by tb_offset because if TB changes via tb_offset changing, + * decrementer does not change, so not directly comparable with TB. + */ now = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL); - next = now + tb_to_ns_round_up(tb_env->decr_freq, value); + next = ns_to_tb(tb_env->decr_freq, now) + value; *nextp = next; /* Adjust timer */ - timer_mod(timer, next); + timer_mod(timer, tb_to_ns_round_up(tb_env->decr_freq, next)); } static inline void _cpu_ppc_store_decr(PowerPCCPU *cpu, target_ulong decr, @@ -1177,12 +1183,15 @@ static void start_stop_pit (CPUPPCState *env, ppc_tb_t *tb_env, int is_excp) } else { trace_ppc4xx_pit_start(ppc40x_timer->pit_reload); now = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL); - next = now + tb_to_ns_round_up(tb_env->decr_freq, - ppc40x_timer->pit_reload); - if (is_excp) - next += tb_env->decr_next - now; + + if (is_excp) { + tb_env->decr_next += ppc40x_timer->pit_reload; + } else { + tb_env->decr_next = ns_to_tb(tb_env->decr_freq, now) + + ppc40x_timer->pit_reload; + } + next = tb_to_ns_round_up(tb_env->decr_freq, tb_env->decr_next); timer_mod(tb_env->decr_timer, next); - tb_env->decr_next = next; } }