Re: [Qemu-devel] [PATCH v12 5/8] virtio-balloon: VIRTIO_BALLOON_F_SG

[Michael S. Tsirkin]

On Sat, Jul 29, 2017 at 08:47:08PM +0800, Wei Wang wrote:
> On 07/29/2017 07:08 AM, Michael S. Tsirkin wrote:
> > On Thu, Jul 27, 2017 at 10:50:11AM +0800, Wei Wang wrote:
> > > > > > OK I thought this over. While we might need these new APIs in
> > > > > > the future, I think that at the moment, there's a way to implement
> > > > > > this feature that is significantly simpler. Just add each s/g
> > > > > > as a separate input buffer.
> > > > > Should it be an output buffer?
> > > > Hypervisor overwrites these pages with zeroes. Therefore it is
> > > > writeable by device: DMA_FROM_DEVICE.
> > > Why would the hypervisor need to zero the buffer?
> > The page is supplied to hypervisor and can lose the value that
> > is there.  That is the definition of writeable by device.
> 
> I think for the free pages, it should be clear that they will be added as
> output buffer to the device, because (as we discussed) they are just hints,
> and some of them may be used by the guest after the report_ API is invoked.
> The device/hypervisor should not use or discard them.

Discarding contents is exactly what you propose doing if
migration is going on, isn't it?

> For the balloon pages, I kind of agree with the existing implementation
> (e.g. inside tell_host()), which uses virtqueue_add_outbuf (instead of
> _add_inbuf()).


This is because it does not pass SGs, it passes weirdly
formatted PA within the buffer.

> I think inbuf should be a buffer which will be written by the device and
> read by the
> driver.

If hypervisor can change it, it's an inbuf. Should not matter
whether driver reads it.

> The cmd buffer put on the vq for the device to send commands can be
> an
> inbuf, I think.
> 
> 
> > 
> > > I think it may only
> > > need to read out the info(base,size).
> > And then do what?
> 
> 
> For the free pages, the info will be used to clear the corresponding "1" in
> the dirty bitmap.
> For balloon pages, they will be made DONTNEED and given to other host
> processes to
> use (the device won't write them, so no need to set "write" when
> virtqueue_map_desc() in
> the device).
> 
> 
> > 
> > > I think it should be like this:
> > > the cmd hdr buffer: input, because the hypervisor would write it to
> > > send a cmd to the guest
> > > the payload buffer: output, for the hypervisor to read the info
> > These should be split.
> > 
> > We have:
> > 
> > 1. request that hypervisor sends to guest, includes ID: input
> > 2. synchronisation header with ID sent by guest: output
> > 3. list of pages: input
> > 
> > 2 and 3 must be on the same VQ. 1 can come on any VQ - reusing stats VQ
> > might make sense.
> 
> I would prefer to make the above item 1 come on the the free page vq,
> because the existing stat_vq doesn't support the cmd hdr.
> Now, I think it is also not necessary to move the existing stat_vq
> implementation to
> a new implementation under the cmd hdr, because
> that new support doesn't make a difference for stats, it will still use its
> stat_vq (the free
> page vq will be used for reporting free pages only)
> 
> What do you think?
> 
> 
> Best,
> Wei