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

[Kai Huang]

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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