linux-kernel - Re: [PATCH v3 05/11] KVM: page track: introduce kvm_page_track_{add,remove}

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Message-ID: <56CC6980.4010904@redhat.com>
Date:	Tue, 23 Feb 2016 15:15:28 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Xiao Guangrong <guangrong.xiao@...ux.intel.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 05/11] KVM: page track: introduce
 kvm_page_track_{add,remove}_page



On 23/02/2016 05:18, Xiao Guangrong wrote:
> 
> 
> On 02/19/2016 07:37 PM, Paolo Bonzini wrote:
>>
>>
>> On 14/02/2016 12:31, Xiao Guangrong wrote:
>>> +    /* does tracking count wrap? */
>>> +    WARN_ON((count > 0) && (val + count < val));
>>
>> This doesn't work, because "val + count" is an int.
> 
> val is 'unsigned short val' and count is 'short', so
> 'val + count' is not int...

Actually, it is.  "val" and "count" are both promoted to int, and the
result of "val + count" is an int!


>>> +void kvm_page_track_add_page(struct kvm *kvm, gfn_t gfn,
>>> +                 enum kvm_page_track_mode mode)
>>> +{
>>> +    struct kvm_memslots *slots;
>>> +    struct kvm_memory_slot *slot;
>>> +    int i;
>>> +
>>> +    for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++) {
>>> +        slots = __kvm_memslots(kvm, i);
>>> +
>>> +        slot = __gfn_to_memslot(slots, gfn);
>>> +        if (!slot)
>>> +            continue;
>>> +
>>> +        spin_lock(&kvm->mmu_lock);
>>> +        kvm_slot_page_track_add_page_nolock(kvm, slot, gfn, mode);
>>> +        spin_unlock(&kvm->mmu_lock);
>>> +    }
>>> +}
>>
>> I don't think it is right to walk all address spaces.  The good news is
> 
> Then we can not track the page in SMM mode, but i think it is not a big
> problem as SMM is invisible to OS (it is expected to not hurting OS) and
> current shadow page in normal spaces can not reflect the changes happend
> in SMM either. So it is okay to only take normal space into account.

I think which address space to track depends on the scenario where
you're using page tracking.  For example, in the shadow case you only
track either SMM or non-SMM depending on the CPU's mode.

For KVM-GT you probably need to track only non-SMM.

>> that you're not using kvm_page_track_{add,remove}_page at all as far as
>> I can see, so you can just remove them.
> 
> kvm_page_track_{add,remove}_page, which hides the mmu specifics (e.g. slot,
> mmu-lock, etc.) and are exported as APIs for other users, are just the
> small wrappers of kvm_slot_page_track_{add,remove}_page_nolock and the
> nolock functions are used in the later patch.
> 
> If you think it is not a good time to export these APIs, i am okay to
> export _nolock functions only in the next version.

Yes, please.

>> Also, when you will need it, I think it's better to move the
>> spin_lock/spin_unlock pair outside the for loop.  With this change,
>> perhaps it's better to leave it to the caller completely---but I cannot
>> say until I see the caller.
> 
> I will remove page tracking in SMM address space, so this is no loop in
> the next version. ;)

Instead please just remove the functions completely.  Functions without
a caller are unnecessary.

>> In the meanwhile, please leave out _nolock from the other functions'
>> name.
> 
> I just wanted to warn the user that these functions are not safe as they
> are not protected by mmu-lock. I will remove these hints if you dislike
> them.

I think there's several other functions that are not protected by
mmu-lock.  You can instead add kerneldoc comments and mention the
locking requirements there.

The common convention in the kernel is to have function take the lock
and call __function.  In this case however there is no "locked" function
yet; if it comes later, we will rename the functions to add "__" in front.

Paolo