From nobody Thu May 15 10:32:19 2025 Delivered-To: importer2@patchew.org Authentication-Results: mx.zohomail.com; spf=pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) smtp.mailfrom=qemu-devel-bounces+importer2=patchew.org@nongnu.org Return-Path: Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) by mx.zohomail.com with SMTPS id 1699538492750448.1449778421663; Thu, 9 Nov 2023 06:01:32 -0800 (PST) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1r15Zn-0005Ut-Ol; Thu, 09 Nov 2023 09:00:09 -0500 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 1r15ZU-00059X-Fc; Thu, 09 Nov 2023 08:59:49 -0500 Received: from isrv.corpit.ru ([86.62.121.231]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1r15ZR-0005Pn-EN; Thu, 09 Nov 2023 08:59:47 -0500 Received: from tsrv.corpit.ru (tsrv.tls.msk.ru [192.168.177.2]) by isrv.corpit.ru (Postfix) with ESMTP id 6E20731BBA; Thu, 9 Nov 2023 16:59:41 +0300 (MSK) Received: from tls.msk.ru (mjt.wg.tls.msk.ru [192.168.177.130]) by tsrv.corpit.ru (Postfix) with SMTP id 6DB40344F3; Thu, 9 Nov 2023 16:59:33 +0300 (MSK) Received: (nullmailer pid 1462766 invoked by uid 1000); Thu, 09 Nov 2023 13:59:33 -0000 From: Michael Tokarev To: qemu-devel@nongnu.org Cc: qemu-stable@nongnu.org, Nicholas Piggin , =?UTF-8?q?C=C3=A9dric=20Le=20Goater?= , Michael Tokarev Subject: [Stable-7.2.7 04/62] hw/ppc: Avoid decrementer rounding errors Date: Thu, 9 Nov 2023 16:58:32 +0300 Message-Id: <20231109135933.1462615-4-mjt@tls.msk.ru> X-Mailer: git-send-email 2.39.2 In-Reply-To: References: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable Received-SPF: pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) client-ip=209.51.188.17; envelope-from=qemu-devel-bounces+importer2=patchew.org@nongnu.org; helo=lists.gnu.org; Received-SPF: pass client-ip=86.62.121.231; envelope-from=mjt@tls.msk.ru; helo=isrv.corpit.ru X-Spam_score_int: -68 X-Spam_score: -6.9 X-Spam_bar: ------ X-Spam_report: (-6.9 / 5.0 requ) BAYES_00=-1.9, RCVD_IN_DNSWL_HI=-5, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=ham 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+importer2=patchew.org@nongnu.org Sender: qemu-devel-bounces+importer2=patchew.org@nongnu.org X-ZM-MESSAGEID: 1699538494040100002 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 =3D now_ns + decr * ns_per_sec / tb_per_sec load: decr =3D (decr_next - now_ns) * tb_per_sec / ns_per_sec load(store): decr =3D decr * ns_per_sec / tb_per_sec * tb_per_sec / ns_per_sec After: store: decr_next =3D now_ns * tb_per_sec / ns_per_sec + decr load: decr =3D decr_next - now_ns * tb_per_sec / ns_per_sec load(store): decr =3D 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: C=C3=A9dric Le Goater (cherry picked from commit 8e0a5ac87800ccc6dd5013f89f27652f4480ab33) Signed-off-by: Michael Tokarev diff --git a/hw/ppc/ppc.c b/hw/ppc/ppc.c index 1996a03b57..9961abc505 100644 --- a/hw/ppc/ppc.c +++ b/hw/ppc/ppc.c @@ -714,16 +714,17 @@ 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 =3D env->tb_env; - int64_t decr, diff; + uint64_t now, n; + int64_t decr; =20 - diff =3D next - qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL); - if (diff >=3D 0) { - decr =3D ns_to_tb(tb_env->decr_freq, diff); - } else if (tb_env->flags & PPC_TIMER_BOOKE) { + now =3D qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL); + n =3D ns_to_tb(tb_env->decr_freq, now); + if (next > n && tb_env->flags & PPC_TIMER_BOOKE) { decr =3D 0; - } else { - decr =3D -ns_to_tb(tb_env->decr_freq, -diff); + } else { + decr =3D next - n; } + trace_ppc_decr_load(decr); =20 return decr; @@ -865,13 +866,18 @@ static void __cpu_ppc_store_decr(PowerPCCPU *cpu, uin= t64_t *nextp, (*lower_excp)(cpu); } =20 - /* 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 chang= ing, + * decrementer does not change, so not directly comparable with TB. + */ now =3D qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL); - next =3D now + tb_to_ns_round_up(tb_env->decr_freq, value); + next =3D ns_to_tb(tb_env->decr_freq, now) + value; *nextp =3D next; =20 /* Adjust timer */ - timer_mod(timer, next); + timer_mod(timer, tb_to_ns_round_up(tb_env->decr_freq, next)); } =20 static inline void _cpu_ppc_store_decr(PowerPCCPU *cpu, target_ulong decr, @@ -1182,12 +1188,15 @@ static void start_stop_pit (CPUPPCState *env, ppc_t= b_t *tb_env, int is_excp) } else { trace_ppc4xx_pit_start(ppc40x_timer->pit_reload); now =3D qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL); - next =3D now + tb_to_ns_round_up(tb_env->decr_freq, - ppc40x_timer->pit_reload); - if (is_excp) - next +=3D tb_env->decr_next - now; + + if (is_excp) { + tb_env->decr_next +=3D ppc40x_timer->pit_reload; + } else { + tb_env->decr_next =3D ns_to_tb(tb_env->decr_freq, now) + + ppc40x_timer->pit_reload; + } + next =3D tb_to_ns_round_up(tb_env->decr_freq, tb_env->decr_next); timer_mod(tb_env->decr_timer, next); - tb_env->decr_next =3D next; } } =20 --=20 2.39.2