RE: [PATCH v30 2/4] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT

[Wang, Wei W]

On Friday, April 6, 2018 2:30 AM, Michael S. Tsirkin wrote:
> On Thu, Apr 05, 2018 at 03:47:28PM +0000, Wang, Wei W wrote:
> > On Thursday, April 5, 2018 10:04 PM, Michael S. Tsirkin wrote:
> > > On Thu, Apr 05, 2018 at 02:05:03AM +0000, Wang, Wei W wrote:
> > > > On Thursday, April 5, 2018 9:12 AM, Michael S. Tsirkin wrote:
> > > > > On Thu, Apr 05, 2018 at 12:30:27AM +0000, Wang, Wei W wrote:
> > > > > > On Wednesday, April 4, 2018 10:08 PM, Michael S. Tsirkin wrote:
> > > > > > > On Wed, Apr 04, 2018 at 10:07:51AM +0800, Wei Wang wrote:
> > > > > > > > On 04/04/2018 02:47 AM, Michael S. Tsirkin wrote:
> > > > > > > > > On Wed, Apr 04, 2018 at 12:10:03AM +0800, Wei Wang wrote:
> > > > > > I'm afraid the driver couldn't be aware if the added hints are
> > > > > > stale or not,
> > > > >
> > > > >
> > > > > No - I mean that driver has code that compares two values and
> > > > > stops reporting. Can one of the values be stale?
> > > >
> > > > The driver compares "vb->cmd_id_use != vb->cmd_id_received" to
> > > > decide if it needs to stop reporting hints, and cmd_id_received is
> > > > what the driver reads from host (host notifies the driver to read
> > > > for the latest value). If host sends a new cmd id, it will notify
> > > > the guest to read again. I'm not sure how that could be a stale
> > > > cmd id (or maybe I misunderstood your point here?)
> > > >
> > > > Best,
> > > > Wei
> > >
> > > The comparison is done in one thread, the update in another one.
> >
> > I think this isn't something that could be solved by adding a lock,
> > unless host waits for the driver's ACK about finishing the update
> > (this is not agreed in the QEMU part discussion).
> >
> > Actually virtio_balloon has F_IOMMU_PLATFORM disabled, maybe we
> don't
> > need to worry about that using DMA api case (we only have gpa added to
> > the vq, and having some entries stay in the vq seems fine). For this
> > feature, I think it would not work with F_IOMMU enabled either.
> 
> Adding a code comment explaining all this might be a good idea.

Thanks, will do.

Best,
Wei