linux-kernel - Re: [PATCH 04/11] KVM: page track: add the framework of guest page tracking

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Date:	Wed, 16 Dec 2015 15:33:18 +0800
From:	Kai Huang <kai.huang@...ux.intel.com>
To:	Xiao Guangrong <guangrong.xiao@...ux.intel.com>,
	pbonzini@...hat.com
Cc:	gleb@...nel.org, mtosatti@...hat.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 04/11] KVM: page track: add the framework of guest page
 tracking



On 12/15/2015 04:46 PM, Xiao Guangrong wrote:
>
>
> On 12/15/2015 03:06 PM, Kai Huang wrote:
>> Hi Guangrong,
>>
>> I am starting to review this series, and should have some comments or 
>> questions, you can determine
>> whether they are valuable :)
>
> Thank you very much for your review and breaking the silent on this 
> patchset. ;)
>
>
>>> +static void page_track_slot_free(struct kvm_memory_slot *slot)
>>> +{
>>> +    int i;
>>> +
>>> +    for (i = 0; i < KVM_PAGE_TRACK_MAX; i++)
>>> +        if (slot->arch.gfn_track[i]) {
>>> +            kvfree(slot->arch.gfn_track[i]);
>>> +            slot->arch.gfn_track[i] = NULL;
>>> +        }
>>> +}
>>> +
>>> +int kvm_page_track_create_memslot(struct kvm_memory_slot *slot,
>>> +                  unsigned long npages)
>>> +{
>>> +    int  i, pages = gfn_to_index(slot->base_gfn + npages - 1,
>>> +                  slot->base_gfn, PT_PAGE_TABLE_LEVEL) + 1;
>>> +
>>> +    for (i = 0; i < KVM_PAGE_TRACK_MAX; i++) {
>>> +        slot->arch.gfn_track[i] = kvm_kvzalloc(pages *
>>> +                        sizeof(*slot->arch.gfn_track[i]));
>>> +        if (!slot->arch.gfn_track[i])
>>> +            goto track_free;
>>> +    }
>>> +
>>> +    return 0;
>>> +
>>> +track_free:
>>> +    page_track_slot_free(slot);
>>> +    return -ENOMEM;
>>> +}
>> Is it necessary to use the 'unsigned long npages' pareameter? In my 
>> understanding you are going to
>
> The type, 'int', is used here as I followed the style of 'struct 
> kvm_lpage_info'.
>
> 4 bytes should be enough to track all users and signed type is good to 
> track
> overflow.
>
>> track all GFNs in the memory slot anyway, right? If you want to keep 
>> npages, I think it's better to
>> add a base_gfn as well otherwise you are assuming you are going to 
>> track the npages GFN starting
>> from slot->base_gfn.
>
> Yes, any page in the memslot may be tracked so that there is a index 
> for every
> page.
>
>>
>>> +
>>> +void kvm_page_track_free_memslot(struct kvm_memory_slot *free,
>>> +                 struct kvm_memory_slot *dont)
>>> +{
>>> +    if (!dont || free->arch.gfn_track != dont->arch.gfn_track)
>>> +        page_track_slot_free(free);
>>> +}
>>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>>> index c04987e..ad4888a 100644
>>> --- a/arch/x86/kvm/x86.c
>>> +++ b/arch/x86/kvm/x86.c
>>> @@ -7838,6 +7838,8 @@ void kvm_arch_free_memslot(struct kvm *kvm, 
>>> struct kvm_memory_slot *free,
>>>               free->arch.lpage_info[i - 1] = NULL;
>>>           }
>>>       }
>>> +
>>> +    kvm_page_track_free_memslot(free, dont);
>>>   }
>>>   int kvm_arch_create_memslot(struct kvm *kvm, struct 
>>> kvm_memory_slot *slot,
>>> @@ -7886,6 +7888,9 @@ int kvm_arch_create_memslot(struct kvm *kvm, 
>>> struct kvm_memory_slot *slot,
>>>           }
>>>       }
>>> +    if (kvm_page_track_create_memslot(slot, npages))
>>> +        goto out_free;
>>> +
>> Looks essentially you are allocating one int for all GFNs of the slot 
>> unconditionally. In my
>> understanding for most of memory slots, we are not going to track 
>> them, so isn't it going to be
>> wasteful of memory?
>>
>
> Yes, hmm... maybe we can make the index as "unsigned short" then 1G 
> memory only needs 512k index
> buffer. It is not so unacceptable.
Those comments are really minor and don't bother on this :)

Thanks,
-Kai

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