Re: [RFC] KVM: arm64: support enabling dirty log graually in small chunks

[zhukeqian]

Hi Jay,

On 2020/3/11 15:34, Zhoujian (jay) wrote:
> 
> 
>> -----Original Message-----
>> From: zhukeqian
>> Sent: Wednesday, March 11, 2020 3:20 PM
>> To: Marc Zyngier <maz@xxxxxxxxxx>
>> Cc: kvmarm@xxxxxxxxxxxxxxxxxxxxx; kvm@xxxxxxxxxxxxxxx;
>> linux-kernel@xxxxxxxxxxxxxxx; linux-arm-kernel@xxxxxxxxxxxxxxxxxxx; Zhoujian (jay)
>> <jianjay.zhou@xxxxxxxxxx>; Sean Christopherson
>> <sean.j.christopherson@xxxxxxxxx>; Paolo Bonzini <pbonzini@xxxxxxxxxx>;
>> James Morse <james.morse@xxxxxxx>; Julien Thierry
>> <julien.thierry.kdev@xxxxxxxxx>; Suzuki K Poulose <suzuki.poulose@xxxxxxx>
>> Subject: Re: [RFC] KVM: arm64: support enabling dirty log graually in small chunks
>>
>> Hi Marc,
>>
>> On 2020/3/10 21:16, Marc Zyngier wrote:
>>> On 2020-03-10 08:26, zhukeqian wrote:
>>>> Hi Marc,
>>>>
>>>> On 2020/3/9 19:45, Marc Zyngier wrote:
>>>>> Kegian,
>>>
>>> [...]
>>>
>>>>> Is there a userspace counterpart to it?
>>>>>
>>>> As this KVM/x86 related changes have not been merged to mainline
>>>> kernel, some little modification is needed on mainline Qemu.
>>>
>>> Could you please point me to these changes?
>> I made some changes locally listed below.
>>
>> However, Qemu can choose to enable KVM_DIRTY_LOG_INITIALLY_SET or not.
>> Here I made no judgement on dirty_log_manual_caps because I just want to
>> verify the optimization of this patch.
>>
>> diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c index
>> 439a4efe52..1611f644a4 100644
>> --- a/accel/kvm/kvm-all.c
>> +++ b/accel/kvm/kvm-all.c
>> @@ -2007,14 +2007,16 @@ static int kvm_init(MachineState *ms)
>>      s->coalesced_pio = s->coalesced_mmio &&
>>                         kvm_check_extension(s,
>> KVM_CAP_COALESCED_PIO);
>>
>> -    s->manual_dirty_log_protect =
>> +    uint64_t dirty_log_manual_caps =
>>          kvm_check_extension(s,
>> KVM_CAP_MANUAL_DIRTY_LOG_PROTECT2);
>> -    if (s->manual_dirty_log_protect) {
>> -        ret = kvm_vm_enable_cap(s,
>> KVM_CAP_MANUAL_DIRTY_LOG_PROTECT2, 0, 1);
>> +    if (dirty_log_manual_caps) {
>> +        ret = kvm_vm_enable_cap(s,
>> KVM_CAP_MANUAL_DIRTY_LOG_PROTECT2, 0,
>> +                                dirty_log_manual_caps);
>>          if (ret) {
>>              warn_report("Trying to enable
>> KVM_CAP_MANUAL_DIRTY_LOG_PROTECT2 "
>>                          "but failed.  Falling back to the legacy mode. ");
>> -            s->manual_dirty_log_protect = false;
>> +        } else {
>> +            s->manual_dirty_log_protect = true;
>>          }
>>      }
> 
> FYI: I had submitted a patch to the Qemu community some days ago:
> https://patchwork.kernel.org/patch/11419191/
This is very helpful, thanks.
> 
>>>
>>>> As I tested this patch on a 128GB RAM Linux VM with no huge pages,
>>>> the time of enabling dirty log will decrease obviously.
>>>
>>> I'm not sure how realistic that is. Not having huge pages tends to
>>> lead to pretty bad performance in general...
>> Sure, this has no effect on guests which are all of huge pages.
>>
>> For my understanding, once a guest has normal pages (maybe are initialized at
>> beginning or dissloved from huge pages), it can benefit from this patch.
> 
> Yes, I agree.
> 
I will send PATCH v1 soon.
> 
> 
> Regards,
> Jay Zhou
> 
> .
>
Thanks,
Keqian