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

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Date:	Tue, 15 Dec 2015 16:46:47 +0800
From:	Xiao Guangrong <guangrong.xiao@...ux.intel.com>
To:	Kai Huang <kai.huang@...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 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.
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