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

[Michael S. Tsirkin]

On Sun, Jul 30, 2017 at 05:59:17AM +0000, Wang, Wei W wrote:
> On Sunday, July 30, 2017 12:23 PM, Michael S. Tsirkin wrote:
> > 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?
> 
> That's actually a different concept. Please let me explain it with this 
> example:
> 
> The hypervisor receives the hint saying the guest PageX is a free page, but 
> as we know, 
> after that report_ API exits, the guest kernel may take PageX to use, so 
> PageX is not free
> page any more. At this time, if the hypervisor writes to the page, that would 
> crash the guest.
> So, I think the cornerstone of this work is that the hypervisor should not 
> touch the
> reported pages.
> 
> Best,
> Wei    

That's a hypervisor implementation detail. From guest point of view,
discarding contents can not be distinguished from writing old contents.