Messages in this thread | | | Subject | Re: [RFC 2/2] KVM: VMX: Enable bus lock VM exit | From | Xiaoyao Li <> | Date | Wed, 1 Jul 2020 22:12:22 +0800 |
| |
On 7/1/2020 8:44 PM, Vitaly Kuznetsov wrote: > Xiaoyao Li <xiaoyao.li@intel.com> writes: > >> On 7/1/2020 5:04 PM, Vitaly Kuznetsov wrote: >>> Chenyi Qiang <chenyi.qiang@intel.com> writes: >> [...] >>>> static const int kvm_vmx_max_exit_handlers = >>>> @@ -6830,6 +6838,13 @@ static fastpath_t vmx_vcpu_run(struct kvm_vcpu *vcpu) >>>> if (unlikely(vmx->exit_reason.failed_vmentry)) >>>> return EXIT_FASTPATH_NONE; >>>> >>>> + /* >>>> + * check the exit_reason to see if there is a bus lock >>>> + * happened in guest. >>>> + */ >>>> + if (vmx->exit_reason.bus_lock_detected) >>>> + handle_bus_lock(vcpu); >>> >>> In case the ultimate goal is to have an exit to userspace on bus lock, >> >> I don't think we will need an exit to userspace on bus lock. See below. >> >>> the two ways to reach handle_bus_lock() are very different: in case >>> we're handling EXIT_REASON_BUS_LOCK we can easily drop to userspace by >>> returning 0 but what are we going to do in case of >>> exit_reason.bus_lock_detected? The 'higher priority VM exit' may require >>> exit to userspace too. So what's the plan? Maybe we can ignore the case >>> when we're exiting to userspace for some other reason as this is slow >>> already and force the exit otherwise? >> >>> And should we actually introduce >>> the KVM_EXIT_BUS_LOCK and a capability to enable it here? >>> >> >> Introducing KVM_EXIT_BUS_LOCK maybe help nothing. No matter >> EXIT_REASON_BUS_LOCK or exit_reason.bus_lock_detected, the bus lock has >> already happened. Exit to userspace cannot prevent bus lock, so what >> userspace can do is recording and counting as what this patch does in >> vcpu->stat.bus_locks. > > Exiting to userspace would allow to implement custom 'throttling' > policies to mitigate the 'noisy neighbour' problem. The simplest would > be to just inject some sleep time. >
So you want an exit to userspace for every bus lock and leave it all to userspace. Yes, it's doable.
As you said, the exit_reason.bus_lock_detected case is the tricky one. We cannot do the similar to extend vcpu->run->exit_reason, this breaks ABI. Maybe we can extend the vcpu->run->flags to indicate bus lock detected for the other exit reason?
| |