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Goldman" To: qemu-devel@nongnu.org Cc: qemu-arm@nongnu.org, rbolshakov@ddn.com, phil@philjordan.eu, agraf@csgraf.de, peter.maydell@linaro.org, "Scott J. Goldman" Subject: [PATCH] target/arm/hvf: Fix WFI halting to stop idle vCPU spinning Date: Thu, 9 Apr 2026 21:47:26 -0700 Message-ID: <20260410044726.61853-1-scottjgo@gmail.com> X-Mailer: git-send-email 2.50.1 MIME-Version: 1.0 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+importer=patchew.org@nongnu.org; helo=lists.gnu.org; Received-SPF: pass client-ip=2607:f8b0:4864:20::132d; envelope-from=scottjgo@gmail.com; helo=mail-dy1-x132d.google.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, FREEMAIL_FROM=0.001, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 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: qemu development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+importer=patchew.org@nongnu.org Sender: qemu-devel-bounces+importer=patchew.org@nongnu.org X-ZohoMail-DKIM: pass (identity @gmail.com) X-ZM-MESSAGEID: 1775796531545154100 Content-Type: text/plain; charset="utf-8" Commit b5f8f77271 ("accel/hvf: Implement WFI without using pselect()") changed hvf_wfi() from blocking the vCPU thread with pselect() to returning EXCP_HLT, intending QEMU's main event loop to handle the idle wait. However, cpu->halted was never set, so cpu_thread_is_idle() always returns false and the vCPU thread spins at 100% CPU per core while the guest is idle. Fix this by: 1. Setting cpu->halted =3D 1 in hvf_wfi() so the vCPU thread sleeps on halt_cond in qemu_process_cpu_events(). 2. Arming a host-side QEMU_CLOCK_HOST timer to fire when the guest's virtual timer (CNTV_CVAL_EL0) would expire. This is necessary because HVF only delivers HV_EXIT_REASON_VTIMER_ACTIVATED during hv_vcpu_run(), which is not called while the CPU is halted. The timer callback mirrors the VTIMER_ACTIVATED handler: it raises the vtimer IRQ through the GIC and marks vtimer_masked, causing the interrupt delivery chain to wake the vCPU via qemu_cpu_kick(). 3. Clearing cpu->halted in hvf_arch_vcpu_exec() when cpu_has_work() indicates a pending interrupt, and cancelling the WFI timer. Fixes: b5f8f77271 ("accel/hvf: Implement WFI without using pselect()") Resolves: https://gitlab.com/qemu-project/qemu/-/issues/959 Signed-off-by: Scott J. Goldman Reviewed-by: Mohamed Mediouni --- include/system/hvf_int.h | 1 + target/arm/hvf/hvf.c | 65 +++++++++++++++++++++++++++++++++++++++- 2 files changed, 65 insertions(+), 1 deletion(-) diff --git a/include/system/hvf_int.h b/include/system/hvf_int.h index 2621164cb2..58fb865eba 100644 --- a/include/system/hvf_int.h +++ b/include/system/hvf_int.h @@ -48,6 +48,7 @@ struct AccelCPUState { hv_vcpu_exit_t *exit; bool vtimer_masked; bool guest_debug_enabled; + struct QEMUTimer *wfi_timer; #endif }; =20 diff --git a/target/arm/hvf/hvf.c b/target/arm/hvf/hvf.c index 5fc8f6bbbd..0e737d3583 100644 --- a/target/arm/hvf/hvf.c +++ b/target/arm/hvf/hvf.c @@ -28,6 +28,7 @@ #include "hw/core/boards.h" #include "hw/core/irq.h" #include "qemu/main-loop.h" +#include "qemu/timer.h" #include "system/cpus.h" #include "arm-powerctl.h" #include "target/arm/cpu.h" @@ -301,6 +302,8 @@ void hvf_arm_init_debug(void) #define TMR_CTL_IMASK (1 << 1) #define TMR_CTL_ISTATUS (1 << 2) =20 +static void hvf_wfi_timer_cb(void *opaque); + static uint32_t chosen_ipa_bit_size; =20 typedef struct HVFVTimer { @@ -1214,6 +1217,9 @@ void hvf_arch_vcpu_destroy(CPUState *cpu) { hv_return_t ret; =20 + timer_free(cpu->accel->wfi_timer); + cpu->accel->wfi_timer =3D NULL; + ret =3D hv_vcpu_destroy(cpu->accel->fd); assert_hvf_ok(ret); } @@ -1352,6 +1358,9 @@ int hvf_arch_init_vcpu(CPUState *cpu) arm_cpu->isar.idregs[ID_AA64MMFR0_EL1_IDX]); assert_hvf_ok(ret); =20 + cpu->accel->wfi_timer =3D timer_new_ns(QEMU_CLOCK_HOST, + hvf_wfi_timer_cb, cpu); + aarch64_add_sme_properties(OBJECT(cpu)); return 0; } @@ -2027,8 +2036,29 @@ static uint64_t hvf_vtimer_val_raw(void) return mach_absolute_time() - hvf_state->vtimer_offset; } =20 +static void hvf_wfi_timer_cb(void *opaque) +{ + CPUState *cpu =3D opaque; + ARMCPU *arm_cpu =3D ARM_CPU(cpu); + + /* + * vtimer expired while the CPU was halted for WFI. + * Mirror HV_EXIT_REASON_VTIMER_ACTIVATED: raise the vtimer + * interrupt and mark as masked so hvf_sync_vtimer() will + * check and unmask when the guest handles it. + * + * The interrupt delivery chain (GIC -> cpu_interrupt -> + * qemu_cpu_kick) wakes the vCPU thread from halt_cond. + */ + qemu_set_irq(arm_cpu->gt_timer_outputs[GTIMER_VIRT], 1); + cpu->accel->vtimer_masked =3D true; +} + static int hvf_wfi(CPUState *cpu) { + uint64_t ctl, cval; + hv_return_t r; + if (cpu_has_work(cpu)) { /* * Don't bother to go into our "low power state" if @@ -2037,6 +2067,35 @@ static int hvf_wfi(CPUState *cpu) return 0; } =20 + /* + * Set up a host-side timer to wake us when the vtimer expires. + * HVF only delivers HV_EXIT_REASON_VTIMER_ACTIVATED during + * hv_vcpu_run(), which we won't call while halted. + */ + r =3D hv_vcpu_get_sys_reg(cpu->accel->fd, HV_SYS_REG_CNTV_CTL_EL0, &ct= l); + assert_hvf_ok(r); + + if ((ctl & TMR_CTL_ENABLE) && !(ctl & TMR_CTL_IMASK)) { + r =3D hv_vcpu_get_sys_reg(cpu->accel->fd, + HV_SYS_REG_CNTV_CVAL_EL0, &cval); + assert_hvf_ok(r); + + uint64_t now =3D hvf_vtimer_val_raw(); + if (cval <=3D now) { + /* Timer already expired, don't halt */ + return 0; + } + + uint64_t delta_ticks =3D cval - now; + mach_timebase_info_data_t timebase; + mach_timebase_info(&timebase); + int64_t delta_ns =3D delta_ticks * timebase.numer / timebase.denom; + int64_t deadline =3D qemu_clock_get_ns(QEMU_CLOCK_HOST) + delta_ns; + + timer_mod(cpu->accel->wfi_timer, deadline); + } + + cpu->halted =3D 1; return EXCP_HLT; } =20 @@ -2332,7 +2391,11 @@ int hvf_arch_vcpu_exec(CPUState *cpu) hv_return_t r; =20 if (cpu->halted) { - return EXCP_HLT; + if (!cpu_has_work(cpu)) { + return EXCP_HLT; + } + cpu->halted =3D 0; + timer_del(cpu->accel->wfi_timer); } =20 flush_cpu_state(cpu); --=20 2.50.1 (Apple Git-155)