[{"id":3688769,"web_url":"http://patchwork.ozlabs.org/comment/3688769/","msgid":"<2eae4f6a-713c-4c4b-af12-aff5c5134d2b@linaro.org>","list_archive_url":null,"date":"2026-05-08T23:14:25","subject":"Re: [PATCH v5 5/7] target/arm: enable event stream on WFE\n instructions","submitter":{"id":72104,"url":"http://patchwork.ozlabs.org/api/people/72104/","name":"Richard Henderson","email":"richard.henderson@linaro.org"},"content":"On 5/5/26 06:20, Alex Bennée wrote:\n> +/*\n> + * Unlike almost everything else that messes with the halt_reason and\n> + * event_register details the timer callbacks are not in the vCPU\n> + * context.\n> + *\n> + * To prevent races we atomically consume a HALT_WFE and set the event\n> + * register. Either way we trigger the an exit event.\n> + */\n> static void arm_wfxt_timer_cb(void *opaque)\n> {\n> ARMCPU *cpu = opaque;\n> CPUState *cs = CPU(cpu);\n> + CPUARMState *env = &cpu->env;\n> +\n> + if (qatomic_cmpxchg(&env->halt_reason, HALT_WFE, NOT_HALTED)) {\n> + qatomic_set(&env->event_register, true);\n> + }\n\nA qatomic_set should be paired with some other qatomic.\n\nThis callback runs in the main loop thread.\n\n> + if (env->event_register) {\n> + env->event_register = false;\n> + return;\n> + }\n\nThis is always run on a vcpu thread.\nIs there a race here? Do you actually need\n\n if (qatomic_xchg(&env->event_register, false))\n\n> @@ -495,32 +586,46 @@ void HELPER(wfe)(CPUARMState *env)\n> #else\n> /*\n> * WFE (Wait For Event) is a hint instruction.\n> - * For Cortex-M (M-profile), we implement the strict architectural behavior:\n> + *\n> * 1. Check the Event Register (set by SEV or SEVONPEND).\n> * 2. If set, clear it and continue (consume the event).\n> */\n> - if (arm_feature(env, ARM_FEATURE_M)) {\n> - CPUState *cs = env_cpu(env);\n> + CPUState *cs = env_cpu(env);\n> + ARMCPU *cpu = env_archcpu(env);\n> + uint32_t excp;\n> + int target_el = check_wfx_trap(env, true, &excp);\n\ntarget_el is in the wrong patch, unused here.\nIt's also being computed too early.\n\n\nr~","headers":{"Return-Path":"","X-Original-To":"incoming@patchwork.ozlabs.org","Delivered-To":"patchwork-incoming@legolas.ozlabs.org","Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (2048-bit key;\n unprotected) header.d=linaro.org header.i=@linaro.org header.a=rsa-sha256\n header.s=google header.b=OhXDr09o;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=nongnu.org\n (client-ip=209.51.188.17; helo=lists1p.gnu.org;\n envelope-from=qemu-devel-bounces+incoming=patchwork.ozlabs.org@nongnu.org;\n receiver=patchwork.ozlabs.org)"],"Received":["from lists1p.gnu.org (lists1p.gnu.org [209.51.188.17])\n\t(using TLSv1.2 with cipher ECDHE-ECDSA-AES256-GCM-SHA384 (256/256 bits))\n\t(No client certificate requested)\n\tby legolas.ozlabs.org (Postfix) with ESMTPS id 4gC4hz1d37z1yCg\n\tfor ; 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