From 19c417ec87a446ffd1a13eeec23226fe30f31b7e Mon Sep 17 00:00:00 2001 From: David Woodhouse Date: Mon, 22 May 2023 20:52:00 +0200 Subject: [PATCH] i386/xen: consistent locking around Xen singleshot timers MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Coverity points out (CID 1507534, 1507968) that we sometimes access env->xen_singleshot_timer_ns under the protection of env->xen_timers_lock and sometimes not. This isn't always an issue. There are two modes for the timers; if the kernel supports the EVTCHN_SEND capability then it handles all the timer hypercalls and delivery internally, and all we use the field for is to get/set the timer as part of the vCPU state via an ioctl(). If the kernel doesn't have that support, then we do all the emulation within qemu, and *those* are the code paths where we actually care about the locking. But it doesn't hurt to be a little bit more consistent and avoid having to explain *why* it's OK. Signed-off-by: David Woodhouse Reviewed-by: Paul Durrant Message-Id: <20230801175747.145906-3-dwmw2@infradead.org> Signed-off-by: Philippe Mathieu-Daudé --- target/i386/kvm/xen-emu.c | 37 +++++++++++++++++++++++++++---------- 1 file changed, 27 insertions(+), 10 deletions(-) diff --git a/target/i386/kvm/xen-emu.c b/target/i386/kvm/xen-emu.c index d7c7eb8d9c..a8146115f0 100644 --- a/target/i386/kvm/xen-emu.c +++ b/target/i386/kvm/xen-emu.c @@ -43,6 +43,7 @@ static void xen_vcpu_singleshot_timer_event(void *opaque); static void xen_vcpu_periodic_timer_event(void *opaque); +static int vcpuop_stop_singleshot_timer(CPUState *cs); #ifdef TARGET_X86_64 #define hypercall_compat32(longmode) (!(longmode)) @@ -466,6 +467,7 @@ void kvm_xen_inject_vcpu_callback_vector(uint32_t vcpu_id, int type) } } +/* Must always be called with xen_timers_lock held */ static int kvm_xen_set_vcpu_timer(CPUState *cs) { X86CPU *cpu = X86_CPU(cs); @@ -483,6 +485,7 @@ static int kvm_xen_set_vcpu_timer(CPUState *cs) static void do_set_vcpu_timer_virq(CPUState *cs, run_on_cpu_data data) { + QEMU_LOCK_GUARD(&X86_CPU(cs)->env.xen_timers_lock); kvm_xen_set_vcpu_timer(cs); } @@ -545,7 +548,6 @@ static void do_vcpu_soft_reset(CPUState *cs, run_on_cpu_data data) env->xen_vcpu_time_info_gpa = INVALID_GPA; env->xen_vcpu_runstate_gpa = INVALID_GPA; env->xen_vcpu_callback_vector = 0; - env->xen_singleshot_timer_ns = 0; memset(env->xen_virq, 0, sizeof(env->xen_virq)); set_vcpu_info(cs, INVALID_GPA); @@ -555,8 +557,13 @@ static void do_vcpu_soft_reset(CPUState *cs, run_on_cpu_data data) INVALID_GPA); if (kvm_xen_has_cap(EVTCHN_SEND)) { kvm_xen_set_vcpu_callback_vector(cs); + + QEMU_LOCK_GUARD(&X86_CPU(cs)->env.xen_timers_lock); + env->xen_singleshot_timer_ns = 0; kvm_xen_set_vcpu_timer(cs); - } + } else { + vcpuop_stop_singleshot_timer(cs); + }; } @@ -1059,6 +1066,10 @@ static int vcpuop_stop_periodic_timer(CPUState *target) return 0; } +/* + * Userspace handling of timer, for older kernels. + * Must always be called with xen_timers_lock held. + */ static int do_set_singleshot_timer(CPUState *cs, uint64_t timeout_abs, bool future, bool linux_wa) { @@ -1086,12 +1097,8 @@ static int do_set_singleshot_timer(CPUState *cs, uint64_t timeout_abs, timeout_abs = now + delta; } - qemu_mutex_lock(&env->xen_timers_lock); - timer_mod_ns(env->xen_singleshot_timer, qemu_now + delta); env->xen_singleshot_timer_ns = now + delta; - - qemu_mutex_unlock(&env->xen_timers_lock); return 0; } @@ -1115,6 +1122,7 @@ static int vcpuop_set_singleshot_timer(CPUState *cs, uint64_t arg) return -EFAULT; } + QEMU_LOCK_GUARD(&X86_CPU(cs)->env.xen_timers_lock); return do_set_singleshot_timer(cs, sst.timeout_abs_ns, !!(sst.flags & VCPU_SSHOTTMR_future), false); @@ -1141,6 +1149,7 @@ static bool kvm_xen_hcall_set_timer_op(struct kvm_xen_exit *exit, X86CPU *cpu, if (unlikely(timeout == 0)) { err = vcpuop_stop_singleshot_timer(CPU(cpu)); } else { + QEMU_LOCK_GUARD(&X86_CPU(cpu)->env.xen_timers_lock); err = do_set_singleshot_timer(CPU(cpu), timeout, false, true); } exit->u.hcall.result = err; @@ -1826,6 +1835,7 @@ int kvm_put_xen_state(CPUState *cs) * If the kernel has EVTCHN_SEND support then it handles timers too, * so the timer will be restored by kvm_xen_set_vcpu_timer() below. */ + QEMU_LOCK_GUARD(&env->xen_timers_lock); if (env->xen_singleshot_timer_ns) { ret = do_set_singleshot_timer(cs, env->xen_singleshot_timer_ns, false, false); @@ -1844,10 +1854,8 @@ int kvm_put_xen_state(CPUState *cs) } if (env->xen_virq[VIRQ_TIMER]) { - ret = kvm_xen_set_vcpu_timer(cs); - if (ret < 0) { - return ret; - } + do_set_vcpu_timer_virq(cs, + RUN_ON_CPU_HOST_INT(env->xen_virq[VIRQ_TIMER])); } return 0; } @@ -1896,6 +1904,15 @@ int kvm_get_xen_state(CPUState *cs) if (ret < 0) { return ret; } + + /* + * This locking is fairly pointless, and is here to appease Coverity. + * There is an unavoidable race condition if a different vCPU sets a + * timer for this vCPU after the value has been read out. But that's + * OK in practice because *all* the vCPUs need to be stopped before + * we set about migrating their state. + */ + QEMU_LOCK_GUARD(&X86_CPU(cs)->env.xen_timers_lock); env->xen_singleshot_timer_ns = va.u.timer.expires_ns; }