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

[Zhoujian (jay)]

> -----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/

> >
> >> 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.



Regards,
Jay Zhou