From: Michael Kelley <mhklinux@xxxxxxxxxxx> vmbus_irq_set_affinity() may issue a MODIFYCHANNEL request to Hyper-V to target a VMBus channel interrupt to a different CPU. While newer versions of Hyper-V send a response to the guest when the change is complete, vmbus_irq_set_affinity() does not wait for the response because it is running with interrupts disabled. So Hyper-V may continue to direct interrupts to the old CPU for a short window after vmbus_irq_set_affinity() completes. This lag is not a problem during normal operation. But if the old CPU is taken offline during that window, Hyper-V may drop the interrupt because the synic in the target CPU is disabled. Dropping the interrupt may cause the VMBus channel to hang. To prevent this, wait for in-process MODIFYCHANNEL requests when taking a CPU offline. On newer versions of Hyper-V, completion can be confirmed by waiting for the response sent by Hyper-V. But on older versions of Hyper-V that don't send a response, wait a fixed interval of time that empirically should be "long enough", as that's the best that can be done. Signed-off-by: Michael Kelley <mhklinux@xxxxxxxxxxx> --- drivers/hv/channel.c | 3 ++ drivers/hv/channel_mgmt.c | 32 ++++-------------- drivers/hv/hv.c | 70 +++++++++++++++++++++++++++++++++++---- drivers/hv/hyperv_vmbus.h | 8 +++++ 4 files changed, 81 insertions(+), 32 deletions(-) diff --git a/drivers/hv/channel.c b/drivers/hv/channel.c index b7920072e243..b7ee95373049 100644 --- a/drivers/hv/channel.c +++ b/drivers/hv/channel.c @@ -246,6 +246,9 @@ int vmbus_send_modifychannel(struct vmbus_channel *channel, u32 target_vp) ret = vmbus_post_msg(&msg, sizeof(msg), false); trace_vmbus_send_modifychannel(&msg, ret); + if (!ret) + vmbus_connection.modchan_sent++; + return ret; } EXPORT_SYMBOL_GPL(vmbus_send_modifychannel); diff --git a/drivers/hv/channel_mgmt.c b/drivers/hv/channel_mgmt.c index da42aaae994e..960a2f0367d8 100644 --- a/drivers/hv/channel_mgmt.c +++ b/drivers/hv/channel_mgmt.c @@ -1465,40 +1465,20 @@ static void vmbus_ongpadl_created(struct vmbus_channel_message_header *hdr) * vmbus_onmodifychannel_response - Modify Channel response handler. * * This is invoked when we received a response to our channel modify request. - * Find the matching request, copy the response and signal the requesting thread. + * Increment the count of responses received. No locking is needed because + * responses are always received on the VMBUS_CONNECT_CPU. */ static void vmbus_onmodifychannel_response(struct vmbus_channel_message_header *hdr) { struct vmbus_channel_modifychannel_response *response; - struct vmbus_channel_msginfo *msginfo; - unsigned long flags; response = (struct vmbus_channel_modifychannel_response *)hdr; + if (response->status) + pr_err("Error status %x in MODIFYCHANNEL response for relid %d\n", + response->status, response->child_relid); + vmbus_connection.modchan_completed++; trace_vmbus_onmodifychannel_response(response); - - /* - * Find the modify msg, copy the response and signal/unblock the wait event. - */ - spin_lock_irqsave(&vmbus_connection.channelmsg_lock, flags); - - list_for_each_entry(msginfo, &vmbus_connection.chn_msg_list, msglistentry) { - struct vmbus_channel_message_header *responseheader = - (struct vmbus_channel_message_header *)msginfo->msg; - - if (responseheader->msgtype == CHANNELMSG_MODIFYCHANNEL) { - struct vmbus_channel_modifychannel *modifymsg; - - modifymsg = (struct vmbus_channel_modifychannel *)msginfo->msg; - if (modifymsg->child_relid == response->child_relid) { - memcpy(&msginfo->response.modify_response, response, - sizeof(*response)); - complete(&msginfo->waitevent); - break; - } - } - } - spin_unlock_irqrestore(&vmbus_connection.channelmsg_lock, flags); } /* diff --git a/drivers/hv/hv.c b/drivers/hv/hv.c index a8ad728354cb..76658dfc5008 100644 --- a/drivers/hv/hv.c +++ b/drivers/hv/hv.c @@ -401,6 +401,56 @@ void hv_synic_disable_regs(unsigned int cpu) disable_percpu_irq(vmbus_irq); } +static void hv_synic_wait_for_modifychannel(int cpu) +{ + int i = 5; + u64 base; + + /* + * If we're on a VMBus version where MODIFYCHANNEL doesn't send acks, + * just sleep for 20 milliseconds and hope that gives Hyper-V enough + * time to process them. Empirical data on recent server-class CPUs + * (both x86 and arm64) shows that the Hyper-V response is typically + * received and processed in the guest within a few hundred + * microseconds. The 20 millisecond wait is somewhat arbitrary and + * intended to give plenty to time in case there are multiple + * MODIFYCHANNEL requests in progress and the host is busy. It's + * the best we can do. + */ + if (vmbus_proto_version < VERSION_WIN10_V5_3) { + usleep_range(20000, 25000); + return; + } + + /* + * Otherwise compare the current value of modchan_completed against + * modchan_sent. If some MODIFYCHANNEL requests have been sent that + * haven't completed, sleep 5 milliseconds and check again. If the + * requests still haven't completed after 5 attempts, output a + * message and proceed anyway. + * + * Hyper-V guarantees to process MODIFYCHANNEL requests in the order + * they are received from the guest, so simply comparing the counts + * is sufficient. + * + * Note that this check may encompass MODIFYCHANNEL requests that are + * unrelated to the CPU that is going offline. But the only effect is + * to potentially wait a little bit longer than necessary. CPUs going + * offline and affinity changes that result in MODIFYCHANNEL are + * relatively rare and it's not worth the complexity to track them more + * precisely. + */ + base = READ_ONCE(vmbus_connection.modchan_sent); + while (READ_ONCE(vmbus_connection.modchan_completed) < base && i) { + usleep_range(5000, 10000); + i--; + } + + if (i == 0) + pr_err("Timed out waiting for MODIFYCHANNEL. CPU %d sent %lld completed %lld\n", + cpu, base, vmbus_connection.modchan_completed); +} + #define HV_MAX_TRIES 3 /* * Scan the event flags page of 'this' CPU looking for any bit that is set. If we find one @@ -485,13 +535,21 @@ int hv_synic_cleanup(unsigned int cpu) /* * channel_found == false means that any channels that were previously * assigned to the CPU have been reassigned elsewhere with a call of - * vmbus_send_modifychannel(). Scan the event flags page looking for - * bits that are set and waiting with a timeout for vmbus_chan_sched() - * to process such bits. If bits are still set after this operation - * and VMBus is connected, fail the CPU offlining operation. + * vmbus_send_modifychannel(). First wait until any MODIFYCHANNEL + * requests have been completed by Hyper-V, after which we know that + * no new bits in the event flags will be set. Then scan the event flags + * page looking for bits that are set and waiting with a timeout for + * vmbus_chan_sched() to process such bits. If bits are still set + * after this operation, fail the CPU offlining operation. */ - if (vmbus_proto_version >= VERSION_WIN10_V4_1 && hv_synic_event_pending()) - return -EBUSY; + if (vmbus_proto_version >= VERSION_WIN10_V4_1) { + hv_synic_wait_for_modifychannel(cpu); + if (hv_synic_event_pending()) { + pr_err("Events pending when trying to offline CPU %d\n", + cpu); + return -EBUSY; + } + } always_cleanup: hv_stimer_legacy_cleanup(cpu); diff --git a/drivers/hv/hyperv_vmbus.h b/drivers/hv/hyperv_vmbus.h index bf35bb40c55e..571b2955b38e 100644 --- a/drivers/hv/hyperv_vmbus.h +++ b/drivers/hv/hyperv_vmbus.h @@ -264,6 +264,14 @@ struct vmbus_connection { struct irq_domain *vmbus_irq_domain; struct irq_chip vmbus_irq_chip; + /* + * VM-wide counts of MODIFYCHANNEL messages sent and completed. + * Used when taking a CPU offline to make sure the relevant + * MODIFYCHANNEL messages have been completed. + */ + u64 modchan_sent; + u64 modchan_completed; + /* * An offer message is handled first on the work_queue, and then * is further handled on handle_primary_chan_wq or -- 2.25.1