linux-kernel - Re: [PATCH v3 07/11] KVM: page track: add notifier support

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Date:	Tue, 23 Feb 2016 12:34:20 +0800
From:	Xiao Guangrong <guangrong.xiao@...ux.intel.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	gleb@...nel.org, mtosatti@...hat.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, kai.huang@...ux.intel.com,
	jike.song@...el.com
Subject: Re: [PATCH v3 07/11] KVM: page track: add notifier support



On 02/19/2016 07:51 PM, Paolo Bonzini wrote:
>
>
> On 14/02/2016 12:31, Xiao Guangrong wrote:

>> +void kvm_page_track_write(struct kvm_vcpu *vcpu, gpa_t gpa, const u8 *new,
>> +			  int bytes)
>> +{
>> +	struct kvm_page_track_notifier_head *head;
>> +	struct kvm_page_track_notifier_node *n;
>> +	int idx;
>> +
>> +	head = &vcpu->kvm->arch.track_notifier_head;
>
> Please check outside SRCU if the notifier list is empty.  If so, there
> is no need to do the (relatively) expensive srcu_read_lock/unlock.
>

Good to me. I will check it by calling hlist_empty() first before holding
the srcu read lock.

> Paolo
>
>> +	idx = srcu_read_lock(&head->track_srcu);
>> +	hlist_for_each_entry_rcu(n, &head->track_notifier_list, node)
>> +		if (n->track_write)
>> +			n->track_write(vcpu, gpa, new, bytes);
>> +	srcu_read_unlock(&head->track_srcu, idx);
>> +}
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index e25ebb7..98019b6 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -4370,6 +4370,7 @@ int emulator_write_phys(struct kvm_vcpu *vcpu, gpa_t gpa,
>>   	if (ret < 0)
>>   		return 0;
>
> A kvm_vcpu_mark_page_dirty is missing here, isn't it?  I can take care
> of it, but it would be great if you double-checked this.  If so, that
> should be fixed in stable kernels too.

No. It's already been handled in emulator_write_phys() -> kvm_vcpu_write_guest()
-> kvm_vcpu_write_guest_page() -> __kvm_write_guest_page().

>
> Can you add a kvm_vcpu_note_page_write(vcpu, gpa, val, bytes) function
> that takes care of calling kvm_vcpu_mark_page_dirty, kvm_mmu_pte_write
> and kvm_page_track-write?
>

After this patchset, kvm_mmu_pte_write is only a static notifier callback called
by kvm_page_track_write().

And the dirty tracking in emulator_write_phys() is handled in a public API (as my
explanation above), in emulator_cmpxchg_emulated is handled by itself. So i think
it is better to leaving dirty tracking to the separate paths, no? :)