Hi Sean,
On 10/16/23 15:48, Sean Christopherson wrote:
> On Mon, Oct 16, 2023, Dongli Zhang wrote:
>> Hi Sean,
>>
>> On 10/16/23 11:49, Sean Christopherson wrote:
>>> Compile tested only, but the below should fix the vCPU hotplug case. Then
>>> someone (not me) just needs to figure out why kvm_xen_shared_info_init() forces
>>> a masterclock update.
>>>
>>> I still think we should clean up the periodic sync code, but I don't think we
>>> need to periodically sync the masterclock.
>>
>> This looks good to me. The core idea is to not update master clock for the
>> synchronized cases.
>>
>>
>> How about the negative value case? I see in the linux code it is still there?
>
> See below.
>
>> (It is out of the scope of my expectation as I do not need to run vCPUs in
>> different tsc freq as host)
>>
>> Thank you very much!
>>
>> Dongli Zhang
>>
>>>
>>> ---
>>> arch/x86/kvm/x86.c | 29 ++++++++++++++++-------------
>>> 1 file changed, 16 insertions(+), 13 deletions(-)
>>>
>>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>>> index c54e1133e0d3..f0a607b6fc31 100644
>>> --- a/arch/x86/kvm/x86.c
>>> +++ b/arch/x86/kvm/x86.c
>>> @@ -2510,26 +2510,29 @@ static inline int gtod_is_based_on_tsc(int mode)
>>> }
>>> #endif
>>>
>>> -static void kvm_track_tsc_matching(struct kvm_vcpu *vcpu)
>>> +static void kvm_track_tsc_matching(struct kvm_vcpu *vcpu, bool new_generation)
>>> {
>>> #ifdef CONFIG_X86_64
>>> - bool vcpus_matched;
>>> struct kvm_arch *ka = &vcpu->kvm->arch;
>>> struct pvclock_gtod_data *gtod = &pvclock_gtod_data;
>>>
>>> - vcpus_matched = (ka->nr_vcpus_matched_tsc + 1 ==
>>> - atomic_read(&vcpu->kvm->online_vcpus));
>>> + /*
>>> + * To use the masterclock, the host clocksource must be based on TSC
>>> + * and all vCPUs must have matching TSCs. Note, the count for matching
>>> + * vCPUs doesn't include the reference vCPU, hence "+1".
>>> + */
>>> + bool use_master_clock = (ka->nr_vcpus_matched_tsc + 1 ==
>>> + atomic_read(&vcpu->kvm->online_vcpus)) &&
>>> + gtod_is_based_on_tsc(gtod->clock.vclock_mode);
>>>
>>> /*
>>> - * Once the masterclock is enabled, always perform request in
>>> - * order to update it.
>>> - *
>>> - * In order to enable masterclock, the host clocksource must be TSC
>>> - * and the vcpus need to have matched TSCs. When that happens,
>>> - * perform request to enable masterclock.
>>> + * Request a masterclock update if the masterclock needs to be toggled
>>> + * on/off, or when starting a new generation and the masterclock is
>>> + * enabled (compute_guest_tsc() requires the masterclock snaphot to be
>>> + * taken _after_ the new generation is created).
>>> */
>>> - if (ka->use_master_clock ||
>>> - (gtod_is_based_on_tsc(gtod->clock.vclock_mode) && vcpus_matched))
>>> + if ((ka->use_master_clock && new_generation) ||
>>> + (ka->use_master_clock != use_master_clock))
>>> kvm_make_request(KVM_REQ_MASTERCLOCK_UPDATE, vcpu);
>>>
>>> trace_kvm_track_tsc(vcpu->vcpu_id, ka->nr_vcpus_matched_tsc,
>>> @@ -2706,7 +2709,7 @@ static void __kvm_synchronize_tsc(struct kvm_vcpu *vcpu, u64 offset, u64 tsc,
>>> vcpu->arch.this_tsc_nsec = kvm->arch.cur_tsc_nsec;
>>> vcpu->arch.this_tsc_write = kvm->arch.cur_tsc_write;
>>>
>>> - kvm_track_tsc_matching(vcpu);
>>> + kvm_track_tsc_matching(vcpu, !matched);
>
> If my analysis of how the negative timestamp occurred is correct, the problematic
> scenario was if cur_tsc_nsec/cur_tsc_write were updated without a masterclock update.
> Passing !matched for @new_generation means that KVM will force a masterclock update
> if cur_tsc_nsec/cur_tsc_write are changed, i.e. prevent the negative timestamp bug.
Thank you very much for the explanation. Now I understand it.
Thanks to the immediate call to kvm_synchronize_tsc() during each vCPU creation ...
kvm_vm_ioctl(KVM_CREATE_VCPU)
-> kvm_vm_ioctl_create_vcpu()
-> kvm_arch_vcpu_postcreate()
-> kvm_synchronize_tsc()
... the local variable "bool use_master_clock" in your patch may always be true.
At that time, the "(ka->use_master_clock != use_master_clock)" returns true.
As a result, we will be able to trigger the KVM_REQ_MASTERCLOCK_UPDATE during
VM creation for each vCPU.
There is still KVM_REQ_MASTERCLOCK_UPDATE for each vCPU during VM creation.
However, there will be no KVM_REQ_MASTERCLOCK_UPDATE for vCPU hot-add.
Thank you very much!
Dongli Zhang
