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

[Xiao Guangrong]

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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