Linux Virtualization - May 2017 - [virtio-dev] Re: [PATCH v9 2/5] virtio-balloon: VIRTIO_BALLOON_F_BALLOON_CHUNKS

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)?

>
> But the code still needs some cleanup.
>
OK. We'll also still to discuss your comments in the patch 05.

Best,
Wei

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?
> 
> > 
> > But the code still needs some cleanup.
> > 
> 
> OK. We'll also still to discuss your comments in the patch 05.
> 
> Best,
> Wei

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[];
};

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();
3) no 4G size limit;

What do you think?

Best,
Wei