lkml.org 
[lkml]   [2020]   [Jul]   [1]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [RFC 2/2] KVM: VMX: Enable bus lock VM exit
From
Date
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?

\
 
 \ /
  Last update: 2020-07-01 16:13    [W:1.823 / U:0.332 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site