On 2013-03-05 09:46, Gleb Natapov wrote: > On Tue, Mar 05, 2013 at 09:24:40AM +0100, Jan Kiszka wrote: >> On 2013-03-05 08:57, Gleb Natapov wrote: >>> On Tue, Mar 05, 2013 at 12:00:10AM +0100, Jan Kiszka wrote: >>>> On 2013-03-04 22:41, Jan Kiszka wrote: >>>>> From: Jan Kiszka <jan.kiszka@xxxxxxxxxxx> >>>>> >>>>> A VCPU sending INIT or SIPI to some other VCPU races for setting the >>>>> remote VCPU's mp_state. When we were unlucky, KVM_MP_STATE_INIT_RECEIVED >>>>> was overwritten by kvm_emulate_halt and, thus, got lost. >>>>> >>>>> Fix this by raising requests on the sender side that will then be >>>>> handled synchronously over the target VCPU context. >>>>> >>>>> Signed-off-by: Jan Kiszka <jan.kiszka@xxxxxxxxxxx> >>>>> --- >>>>> >>>>> Changes in v2: >>>>> - check transition to INIT_RECEIVED in vcpu_enter_guest >>>>> - removed return value of kvm_check_init_and_sipi - caller has to >>>>> check for relevant transition afterward >>>>> - add write barrier after setting sipi_vector >>>>> >>>>> arch/x86/kvm/lapic.c | 11 ++++++----- >>>>> arch/x86/kvm/x86.c | 15 +++++++++++++++ >>>>> include/linux/kvm_host.h | 2 ++ >>>>> 3 files changed, 23 insertions(+), 5 deletions(-) >>>>> >>>>> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c >>>>> index 02b51dd..7986c9f 100644 >>>>> --- a/arch/x86/kvm/lapic.c >>>>> +++ b/arch/x86/kvm/lapic.c >>>>> @@ -731,8 +731,7 @@ static int __apic_accept_irq(struct kvm_lapic *apic, int delivery_mode, >>>>> case APIC_DM_INIT: >>>>> if (!trig_mode || level) { >>>>> result = 1; >>>>> - vcpu->arch.mp_state = KVM_MP_STATE_INIT_RECEIVED; >>>>> - kvm_make_request(KVM_REQ_EVENT, vcpu); >>>>> + kvm_make_request(KVM_REQ_INIT, vcpu); >>>>> kvm_vcpu_kick(vcpu); >>>>> } else { >>>>> apic_debug("Ignoring de-assert INIT to vcpu %d\n", >>>>> @@ -743,11 +742,13 @@ static int __apic_accept_irq(struct kvm_lapic *apic, int delivery_mode, >>>>> case APIC_DM_STARTUP: >>>>> apic_debug("SIPI to vcpu %d vector 0x%02x\n", >>>>> vcpu->vcpu_id, vector); >>>>> - if (vcpu->arch.mp_state == KVM_MP_STATE_INIT_RECEIVED) { >>>>> + if (vcpu->arch.mp_state == KVM_MP_STATE_INIT_RECEIVED || >>>>> + test_bit(KVM_REQ_INIT, &vcpu->requests)) { >>>>> result = 1; >>>>> vcpu->arch.sipi_vector = vector; >>>>> - vcpu->arch.mp_state = KVM_MP_STATE_SIPI_RECEIVED; >>>>> - kvm_make_request(KVM_REQ_EVENT, vcpu); >>>>> + /* make sure sipi_vector is visible for the receiver */ >>>>> + smp_wmb(); >>>>> + kvm_make_request(KVM_REQ_SIPI, vcpu); >>>>> kvm_vcpu_kick(vcpu); >>>>> } >>>>> break; >>>>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c >>>>> index d0cf737..0be04b9 100644 >>>>> --- a/arch/x86/kvm/x86.c >>>>> +++ b/arch/x86/kvm/x86.c >>>>> @@ -5641,6 +5641,15 @@ static void update_eoi_exitmap(struct kvm_vcpu *vcpu) >>>>> kvm_x86_ops->load_eoi_exitmap(vcpu, eoi_exit_bitmap); >>>>> } >>>>> >>>>> +static void kvm_check_init_and_sipi(struct kvm_vcpu *vcpu) >>>>> +{ >>>>> + if (kvm_check_request(KVM_REQ_INIT, vcpu)) >>>>> + vcpu->arch.mp_state = KVM_MP_STATE_INIT_RECEIVED; >>>> >>>> And here is a small race between clearing REQ_INIT and setting >>>> INIT_RECEIVED. It can make the LAPIC drop the SIPI incorrectly. Need to >>>> break up test and clear, doing the clear after mp_state update. Yeah... >>>> >>> You also need to call kvm_check_init_and_sipi() in >>> kvm_arch_vcpu_ioctl_get_mpstate(), >> >> Indeed. >> >>> which means you now have three places >>> where you transfer INIT/SIPI state from requests to mp_state. All the >>> problems arise from the fact that now you have two places where you >>> are storing current state. >> >> Not at all. I'm keeping the state in a single place, mp_state. I just >> have to make sure that I do not loose asynchronous events - what INIT >> and SIPI are. >> > As evident from this code: > + if (vcpu->arch.mp_state == KVM_MP_STATE_INIT_RECEIVED || > + test_bit(KVM_REQ_INIT, &vcpu->requests)) { > the state is in two places. That's just to protect the content of sipi_vector during delivery. We could drop the complete if clause if we protected that variable differently. > >>> To overcome this we can either deprecated >>> KVM_MP_STATE_INIT_RECEIVED/KVM_MP_STATE_SIPI_RECEIVED values for mp_state >>> (use it only for migration purposes) and use separate state in APIC >>> to hold those event, like with nmi, or why not go with Paolo's simple >>> cmpxchg one? >> >> We need to replace most, if not all, manipulations of mp_state with >> cmpxchg, verifying the state transitions there. And the request-based >> approach still looks cleaner to me when it comes to implementing INIT >> handling for nested modes. That will just trivially hook into >> kvm_check_init_and_sipi. >> > The mp_state changes are rare, do not see the problem replacing all > state changes with cmpxchg. I do not like request-based approach as > implemented since we keep state in two places and constantly sync it > back. And I like it more as it avoids spurious state changes toward INIT. That will happen if we misuse mp_state for event signaling, like we do so far, having to fix it up later again because the INIT event turned out to become an INIT VM-exit. Jan
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