[{"id":3675653,"web_url":"http://patchwork.ozlabs.org/comment/3675653/","msgid":"","list_archive_url":null,"date":"2026-04-10T05:06:18","subject":"Re: [PATCH] target/arm/hvf: Fix WFI halting to stop idle vCPU\n spinning","submitter":{"id":91318,"url":"http://patchwork.ozlabs.org/api/people/91318/","name":"Mohamed Mediouni","email":"mohamed@unpredictable.fr"},"content":"> On 10. Apr 2026, at 06:47, Scott J. Goldman wrote:\n> \n> Commit b5f8f77271 (\"accel/hvf: Implement WFI without using pselect()\")\n> changed hvf_wfi() from blocking the vCPU thread with pselect() to\n> returning EXCP_HLT, intending QEMU's main event loop to handle the\n> idle wait. However, cpu->halted was never set, so cpu_thread_is_idle()\n> always returns false and the vCPU thread spins at 100% CPU per core\n> while the guest is idle.\n> \n> Fix this by:\n> \n> 1. Setting cpu->halted = 1 in hvf_wfi() so the vCPU thread sleeps on\n> halt_cond in qemu_process_cpu_events().\n> \n> 2. Arming a host-side QEMU_CLOCK_HOST timer to fire when the guest's\n> virtual timer (CNTV_CVAL_EL0) would expire. This is necessary\n> because HVF only delivers HV_EXIT_REASON_VTIMER_ACTIVATED during\n> hv_vcpu_run(), which is not called while the CPU is halted. The\n> timer callback mirrors the VTIMER_ACTIVATED handler: it raises the\n> vtimer IRQ through the GIC and marks vtimer_masked, causing the\n> interrupt delivery chain to wake the vCPU via qemu_cpu_kick().\n> \n> 3. Clearing cpu->halted in hvf_arch_vcpu_exec() when cpu_has_work()\n> indicates a pending interrupt, and cancelling the WFI timer.\n> \n> Fixes: b5f8f77271 (\"accel/hvf: Implement WFI without using pselect()\")\n> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/959\n> Signed-off-by: Scott J. Goldman \n\nHi,\n\nReviewed-by: Mohamed Mediouni \n> ---\n> include/system/hvf_int.h | 1 +\n> target/arm/hvf/hvf.c | 65 +++++++++++++++++++++++++++++++++++++++-\n> 2 files changed, 65 insertions(+), 1 deletion(-)\n> \n> diff --git a/include/system/hvf_int.h b/include/system/hvf_int.h\n> index 2621164cb2..58fb865eba 100644\n> --- a/include/system/hvf_int.h\n> +++ b/include/system/hvf_int.h\n> @@ -48,6 +48,7 @@ struct AccelCPUState {\n> hv_vcpu_exit_t *exit;\n> bool vtimer_masked;\n> bool guest_debug_enabled;\n> + struct QEMUTimer *wfi_timer;\n> #endif\n> };\n> \n> diff --git a/target/arm/hvf/hvf.c b/target/arm/hvf/hvf.c\n> index 5fc8f6bbbd..0e737d3583 100644\n> --- a/target/arm/hvf/hvf.c\n> +++ b/target/arm/hvf/hvf.c\n> @@ -28,6 +28,7 @@\n> #include \"hw/core/boards.h\"\n> #include \"hw/core/irq.h\"\n> #include \"qemu/main-loop.h\"\n> +#include \"qemu/timer.h\"\n> #include \"system/cpus.h\"\n> #include \"arm-powerctl.h\"\n> #include \"target/arm/cpu.h\"\n> @@ -301,6 +302,8 @@ void hvf_arm_init_debug(void)\n> #define TMR_CTL_IMASK (1 << 1)\n> #define TMR_CTL_ISTATUS (1 << 2)\n> \n> +static void hvf_wfi_timer_cb(void *opaque);\n> +\n> static uint32_t chosen_ipa_bit_size;\n> \n> typedef struct HVFVTimer {\n> @@ -1214,6 +1217,9 @@ void hvf_arch_vcpu_destroy(CPUState *cpu)\n> {\n> hv_return_t ret;\n> \n> + timer_free(cpu->accel->wfi_timer);\n> + cpu->accel->wfi_timer = NULL;\n> +\n> ret = hv_vcpu_destroy(cpu->accel->fd);\n> assert_hvf_ok(ret);\n> }\n> @@ -1352,6 +1358,9 @@ int hvf_arch_init_vcpu(CPUState *cpu)\n> arm_cpu->isar.idregs[ID_AA64MMFR0_EL1_IDX]);\n> assert_hvf_ok(ret);\n> \n> + cpu->accel->wfi_timer = timer_new_ns(QEMU_CLOCK_HOST,\n> + hvf_wfi_timer_cb, cpu);\n> +\n> aarch64_add_sme_properties(OBJECT(cpu));\n> return 0;\n> }\n> @@ -2027,8 +2036,29 @@ static uint64_t hvf_vtimer_val_raw(void)\n> return mach_absolute_time() - hvf_state->vtimer_offset;\n> }\n> \n> +static void hvf_wfi_timer_cb(void *opaque)\n> +{\n> + CPUState *cpu = opaque;\n> + ARMCPU *arm_cpu = ARM_CPU(cpu);\n> +\n> + /*\n> + * vtimer expired while the CPU was halted for WFI.\n> + * \t\n> + * interrupt and mark as masked so hvf_sync_vtimer() will\n> + * check and unmask when the guest handles it.\n> + *\n> + * The interrupt delivery chain (GIC -> cpu_interrupt ->\n> + * qemu_cpu_kick) wakes the vCPU thread from halt_cond.\n> + */\n> + qemu_set_irq(arm_cpu->gt_timer_outputs[GTIMER_VIRT], 1);\n> + cpu->accel->vtimer_masked = true;\n> +}\n> +\n> static int hvf_wfi(CPUState *cpu)\n> {\n> + uint64_t ctl, cval;\n> + hv_return_t r;\n> +\n> if (cpu_has_work(cpu)) {\n> /*\n> * Don't bother to go into our \"low power state\" if\n> @@ -2037,6 +2067,35 @@ static int hvf_wfi(CPUState *cpu)\n> return 0;\n> }\n> \n> + /*\n> + * Set up a host-side timer to wake us when the vtimer expires.\n> + * HVF only delivers HV_EXIT_REASON_VTIMER_ACTIVATED during\n> + * hv_vcpu_run(), which we won't call while halted.\n> + */\n> + r = hv_vcpu_get_sys_reg(cpu->accel->fd, HV_SYS_REG_CNTV_CTL_EL0, &ctl);\n> + assert_hvf_ok(r);\n> +\n> + if ((ctl & TMR_CTL_ENABLE) && !(ctl & TMR_CTL_IMASK)) {\n> + r = hv_vcpu_get_sys_reg(cpu->accel->fd,\n> + HV_SYS_REG_CNTV_CVAL_EL0, &cval);\n> + assert_hvf_ok(r);\n> +\n> + uint64_t now = hvf_vtimer_val_raw();\n> + if (cval <= now) {\n> + /* Timer already expired, don't halt */\n> + return 0;\n> + }\n> +\n> + uint64_t delta_ticks = cval - now;\n\nWe have CNTFRQ_EL0 in cpu->gt_cntfrq_hz but this works too\nas XNU today tries to take care that host and VM run with\nthe same CNTFRQ_EL0\n\n> + mach_timebase_info_data_t timebase;\n> + mach_timebase_info(&timebase);\n> + int64_t delta_ns = delta_ticks * timebase.numer / timebase.denom;\n> + int64_t deadline = qemu_clock_get_ns(QEMU_CLOCK_HOST) + delta_ns;\n> +\n> + timer_mod(cpu->accel->wfi_timer, deadline);\n> + }\n> +\n> + cpu->halted = 1;\n> return EXCP_HLT;\n> }\n> \n> @@ -2332,7 +2391,11 @@ int hvf_arch_vcpu_exec(CPUState *cpu)\n> hv_return_t r;\n> \n> if (cpu->halted) {\n> - return EXCP_HLT;\n> + if (!cpu_has_work(cpu)) {\n> + return EXCP_HLT;\n> + }\n> + cpu->halted = 0;\n> + timer_del(cpu->accel->wfi_timer);\n> }\n> \n> flush_cpu_state(cpu);\n> -- \n> 2.50.1 (Apple Git-155)\n> \n>","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=unpredictable.fr header.i=@unpredictable.fr\n header.a=rsa-sha256 header.s=sig1 header.b=QsXvj74s;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=nongnu.org\n (client-ip=209.51.188.17; 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Goldman","email":"scottjgo@gmail.com"},"content":"On Thu Apr 9, 2026 at 10:06 PM PDT, Mohamed Mediouni wrote:\n>\n>\n>> On 10. Apr 2026, at 06:47, Scott J. Goldman wrote:\n>> \n>> Commit b5f8f77271 (\"accel/hvf: Implement WFI without using pselect()\")\n>> changed hvf_wfi() from blocking the vCPU thread with pselect() to\n>> returning EXCP_HLT, intending QEMU's main event loop to handle the\n>> idle wait. However, cpu->halted was never set, so cpu_thread_is_idle()\n>> always returns false and the vCPU thread spins at 100% CPU per core\n>> while the guest is idle.\n>> \n>> Fix this by:\n>> \n>> 1. Setting cpu->halted = 1 in hvf_wfi() so the vCPU thread sleeps on\n>> halt_cond in qemu_process_cpu_events().\n>> \n>> 2. Arming a host-side QEMU_CLOCK_HOST timer to fire when the guest's\n>> virtual timer (CNTV_CVAL_EL0) would expire. This is necessary\n>> because HVF only delivers HV_EXIT_REASON_VTIMER_ACTIVATED during\n>> hv_vcpu_run(), which is not called while the CPU is halted. The\n>> timer callback mirrors the VTIMER_ACTIVATED handler: it raises the\n>> vtimer IRQ through the GIC and marks vtimer_masked, causing the\n>> interrupt delivery chain to wake the vCPU via qemu_cpu_kick().\n>> \n>> 3. Clearing cpu->halted in hvf_arch_vcpu_exec() when cpu_has_work()\n>> indicates a pending interrupt, and cancelling the WFI timer.\n>> \n>> Fixes: b5f8f77271 (\"accel/hvf: Implement WFI without using pselect()\")\n>> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/959\n>> Signed-off-by: Scott J. Goldman \n>\n> Hi,\n>\n> Reviewed-by: Mohamed Mediouni \n>> ---\n>> include/system/hvf_int.h | 1 +\n>> target/arm/hvf/hvf.c | 65 +++++++++++++++++++++++++++++++++++++++-\n>> 2 files changed, 65 insertions(+), 1 deletion(-)\n>> \n>> diff --git a/include/system/hvf_int.h b/include/system/hvf_int.h\n>> index 2621164cb2..58fb865eba 100644\n>> --- a/include/system/hvf_int.h\n>> +++ b/include/system/hvf_int.h\n>> @@ -48,6 +48,7 @@ struct AccelCPUState {\n>> hv_vcpu_exit_t *exit;\n>> bool vtimer_masked;\n>> bool guest_debug_enabled;\n>> + struct QEMUTimer *wfi_timer;\n>> #endif\n>> };\n>> \n>> diff --git a/target/arm/hvf/hvf.c b/target/arm/hvf/hvf.c\n>> index 5fc8f6bbbd..0e737d3583 100644\n>> --- a/target/arm/hvf/hvf.c\n>> +++ b/target/arm/hvf/hvf.c\n>> @@ -28,6 +28,7 @@\n>> #include \"hw/core/boards.h\"\n>> #include \"hw/core/irq.h\"\n>> #include \"qemu/main-loop.h\"\n>> +#include \"qemu/timer.h\"\n>> #include \"system/cpus.h\"\n>> #include \"arm-powerctl.h\"\n>> #include \"target/arm/cpu.h\"\n>> @@ -301,6 +302,8 @@ void hvf_arm_init_debug(void)\n>> #define TMR_CTL_IMASK (1 << 1)\n>> #define TMR_CTL_ISTATUS (1 << 2)\n>> \n>> +static void hvf_wfi_timer_cb(void *opaque);\n>> +\n>> static uint32_t chosen_ipa_bit_size;\n>> \n>> typedef struct HVFVTimer {\n>> @@ -1214,6 +1217,9 @@ void hvf_arch_vcpu_destroy(CPUState *cpu)\n>> {\n>> hv_return_t ret;\n>> \n>> + timer_free(cpu->accel->wfi_timer);\n>> + cpu->accel->wfi_timer = NULL;\n>> +\n>> ret = hv_vcpu_destroy(cpu->accel->fd);\n>> assert_hvf_ok(ret);\n>> }\n>> @@ -1352,6 +1358,9 @@ int hvf_arch_init_vcpu(CPUState *cpu)\n>> arm_cpu->isar.idregs[ID_AA64MMFR0_EL1_IDX]);\n>> assert_hvf_ok(ret);\n>> \n>> + cpu->accel->wfi_timer = timer_new_ns(QEMU_CLOCK_HOST,\n>> + hvf_wfi_timer_cb, cpu);\n>> +\n>> aarch64_add_sme_properties(OBJECT(cpu));\n>> return 0;\n>> }\n>> @@ -2027,8 +2036,29 @@ static uint64_t hvf_vtimer_val_raw(void)\n>> return mach_absolute_time() - hvf_state->vtimer_offset;\n>> }\n>> \n>> +static void hvf_wfi_timer_cb(void *opaque)\n>> +{\n>> + CPUState *cpu = opaque;\n>> + ARMCPU *arm_cpu = ARM_CPU(cpu);\n>> +\n>> + /*\n>> + * vtimer expired while the CPU was halted for WFI.\n>> + * \t\n>> + * interrupt and mark as masked so hvf_sync_vtimer() will\n>> + * check and unmask when the guest handles it.\n>> + *\n>> + * The interrupt delivery chain (GIC -> cpu_interrupt ->\n>> + * qemu_cpu_kick) wakes the vCPU thread from halt_cond.\n>> + */\n>> + qemu_set_irq(arm_cpu->gt_timer_outputs[GTIMER_VIRT], 1);\n>> + cpu->accel->vtimer_masked = true;\n>> +}\n>> +\n>> static int hvf_wfi(CPUState *cpu)\n>> {\n>> + uint64_t ctl, cval;\n>> + hv_return_t r;\n>> +\n>> if (cpu_has_work(cpu)) {\n>> /*\n>> * Don't bother to go into our \"low power state\" if\n>> @@ -2037,6 +2067,35 @@ static int hvf_wfi(CPUState *cpu)\n>> return 0;\n>> }\n>> \n>> + /*\n>> + * Set up a host-side timer to wake us when the vtimer expires.\n>> + * HVF only delivers HV_EXIT_REASON_VTIMER_ACTIVATED during\n>> + * hv_vcpu_run(), which we won't call while halted.\n>> + */\n>> + r = hv_vcpu_get_sys_reg(cpu->accel->fd, HV_SYS_REG_CNTV_CTL_EL0, &ctl);\n>> + assert_hvf_ok(r);\n>> +\n>> + if ((ctl & TMR_CTL_ENABLE) && !(ctl & TMR_CTL_IMASK)) {\n>> + r = hv_vcpu_get_sys_reg(cpu->accel->fd,\n>> + HV_SYS_REG_CNTV_CVAL_EL0, &cval);\n>> + assert_hvf_ok(r);\n>> +\n>> + uint64_t now = hvf_vtimer_val_raw();\n>> + if (cval <= now) {\n>> + /* Timer already expired, don't halt */\n>> + return 0;\n>> + }\n>> +\n>> + uint64_t delta_ticks = cval - now;\n>\n> We have CNTFRQ_EL0 in cpu->gt_cntfrq_hz but this works too\n> as XNU today tries to take care that host and VM run with\n> the same CNTFRQ_EL0\n>\n\nAh, I can post a follow-up.\n\n>> + mach_timebase_info_data_t timebase;\n>> + mach_timebase_info(&timebase);\n>> + int64_t delta_ns = delta_ticks * timebase.numer / timebase.denom;\n>> + int64_t deadline = qemu_clock_get_ns(QEMU_CLOCK_HOST) + delta_ns;\n>> +\n>> + timer_mod(cpu->accel->wfi_timer, deadline);\n>> + }\n>> +\n>> + cpu->halted = 1;\n>> return EXCP_HLT;\n>> }\n>> \n>> @@ -2332,7 +2391,11 @@ int hvf_arch_vcpu_exec(CPUState *cpu)\n>> hv_return_t r;\n>> \n>> if (cpu->halted) {\n>> - return EXCP_HLT;\n>> + if (!cpu_has_work(cpu)) {\n>> + return EXCP_HLT;\n>> + }\n>> + cpu->halted = 0;\n>> + timer_del(cpu->accel->wfi_timer);\n>> }\n>> \n>> flush_cpu_state(cpu);\n>> -- \n>> 2.50.1 (Apple Git-155)\n>> \n>> \n\nThanks,\n-sjg","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=gmail.com header.i=@gmail.com header.a=rsa-sha256\n header.s=20251104 header.b=s0gY8Xdd;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=nongnu.org\n (client-ip=209.51.188.17; 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charset=UTF-8","Date":"Thu, 09 Apr 2026 22:28:42 -0700","Message-Id":"","Cc":", , ,\n , , ","Subject":"Re: [PATCH] target/arm/hvf: Fix WFI halting to stop idle vCPU\n spinning","From":"\"Scott J. Goldman\" ","To":"\"Mohamed Mediouni\" , \"Scott J. 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