Re: [Qemu-devel] [virtio-dev] Re: [PATCH v9 2/5] virtio-balloon: VIRTIO_BALLOON_F_BALLOON_CHUNKS

[Wei Wang]

On 05/09/2017 01:40 AM, Michael S. Tsirkin wrote:
> On Sun, May 07, 2017 at 04:19:28AM +0000, Wang, Wei W wrote:
> > On 05/06/2017 06:26 AM, Michael S. Tsirkin wrote:
> > > On Thu, Apr 27, 2017 at 02:31:49PM +0800, Wei Wang wrote:
> > > > On 04/27/2017 07:20 AM, Michael S. Tsirkin wrote:
> > > > > On Wed, Apr 26, 2017 at 11:03:34AM +0000, Wang, Wei W wrote:
> > > > > > Hi Michael, could you please give some feedback?
> > > > > I'm sorry, I'm not sure feedback on what you are requesting.
> > > > Oh, just some trivial things (e.g. use a field in the header,
> > > > hdr->chunks to indicate the number of chunks in the payload) that
> > > > wasn't confirmed.
> > > >
> > > > I will prepare the new version with fixing the agreed issues, and we
> > > > can continue to discuss those parts if you still find them improper.
> > > >
> > > >
> > > > > The interface looks reasonable now, even though there's a way to
> > > > > make it even simpler if we can limit chunk size to 2G (in fact 4G -
> > > > > 1). Do you think we can live with this limitation?
> > > > Yes, I think we can. So, is it good to change to use the previous
> > > > 64-bit chunk format (52-bit base + 12-bit size)?
> > > This isn't what I meant. virtio ring has descriptors with a 64 bit address and 
> > > 32 bit
> > > size.
> > >
> > > If size < 4g is not a significant limitation, why not just use that to pass
> > > address/size in a standard s/g list, possibly using INDIRECT?
> > OK, I see your point, thanks. Post the two options here for an analysis:
> > Option1 (what we have now):
> > struct virtio_balloon_page_chunk {
> >          __le64 chunk_num;
> >          struct virtio_balloon_page_chunk_entry entry[];
> > };
> > Option2:
> > struct virtio_balloon_page_chunk {
> >          __le64 chunk_num;
> >          struct scatterlist entry[];
> > };
> This isn't what I meant really :) I meant vring_desc.

OK. Repost the code change:

Option2:
struct virtio_balloon_page_chunk {
        __le64 chunk_num;
        struct ving_desc entry[];
};

We pre-allocate a table of desc, and each desc is used to hold a chunk.

In that case, the virtqueue_add() function, which deals with sg, is not
usable for us. We may need to add a new one,
virtqueue_add_indirect_desc(),
to add a pre-allocated indirect descriptor table to vring.


> > I don't have an issue to change it to Option2, but I would prefer Option1,
> > because I think there is no be obvious difference between the two options,
> > while Option1 appears to have little advantages here:
> > 1) "struct virtio_balloon_page_chunk_entry" has smaller size than
> > "struct scatterlist", so the same size of allocated page chunk buffer
> > can hold more entry[] using Option1;
> > 2) INDIRECT needs on demand kmalloc();
> Within alloc_indirect?  We can fix that with a separate patch.
>
>
> > 3) no 4G size limit;
> Do you see lots of >=4g chunks in practice?
It wouldn't be much in practice, but we still need the extra code to
handle the case - break larger chunks into less-than 4g ones.

Best,
Wei