Re: [Qemu-devel] [PATCH v6 kernel 3/5] virtio-balloon: speed up inflate/deflate process

[Michael S. Tsirkin]

On Thu, Jan 19, 2017 at 01:44:36AM +0000, Li, Liang Z wrote:
> > > > > +             *range = cpu_to_le64((base_pfn <<
> > > > > +                             VIRTIO_BALLOON_NR_PFN_BITS) | 0);
> > > > > +             *(range + 1) = cpu_to_le64(pages);
> > > > > +             vb->resp_pos += 2;
> > > >
> > > > Pls use structs for this kind of stuff.
> > >
> > > I am not sure if you mean to use
> > >
> > > struct  range {
> > >   __le64 pfn: 52;
> > >   __le64 nr_page: 12
> > > }
> > > Instead of the shift operation?
> > 
> > Not just that. You want to add a pages field as well.
> > 
> 
> pages field? Could you give more hints?

Well look how you are formatting it manually above.
There is clearly a structure with two 64 bit fields.
First one includes pfn and 0 (no idea why does | 0 make
sense but that's a separate issue).
Second one includes the pages value.


> > Generally describe the format in the header in some way so host and guest
> > can easily stay in sync.
> 
> 'VIRTIO_BALLOON_NR_PFN_BITS' is for this purpose and it will be passed to the
> related function in page_alloc.c as a parameter.
> 
> Thanks!
> Liang
> > All the pointer math and void * means we get zero type safety and I'm not
> > happy about it.
> > 
> > It's not good that virtio format seeps out to page_alloc anyway.
> > If unavoidable it is not a good idea to try to hide this fact, people will 
> > assume
> > they can change the format at will.
> > 
> > --
> > MST