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

[Michael S. Tsirkin]

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.

> 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?

> What do you think?
> 
> Best,
> Wei
> 
>

OTOH using existing vring APIs handles things like DMA transparently.


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