Re: [Qemu-devel] [PATCH v7 4/5] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT

[Michael S. Tsirkin]

On Fri, Jun 08, 2018 at 09:37:23AM +0800, Peter Xu wrote:
> > > Asked since I see this in virtio spec (v1.0, though I guess it won't
> > > change) in chapter "2.2.1 Driver Requirements: Feature Bits":
> > > 
> > > "The driver MUST NOT accept a feature which the device did not offer"
> > > 
> > > Then I'm curious what would happen if:
> > > 
> > > - a emulator (not QEMU) only offered F_FREE_PAGE_HINT, not F_POISON
> > > - a guest that enabled PAGE_POISON
> > > 
> > > Then how the driver could tell the host that PAGE_POISON is enabled
> > > considering that guest should never set that feature bit if the
> > > emulation code didn't provide it?

It wouldn't. It just has to deal with the fact that host can discard
writes to hinted pages. Right now driver deals with it simply by
disabling F_FREE_PAGE_HINT.

-- 
MST