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.
