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

[Peter Xu]

On Fri, Jun 08, 2018 at 04:58:21AM +0300, Michael S. Tsirkin wrote:
> 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.

Ah I see.  Thanks Michael.

Then it seems to me that it's more important to implement the F_POISON
along with where it is declared since otherwise it'll be a real broken
(device declares F_POISON, guest assumes it can handle the POISON so
guest will enable FREE_PAGE_HINT, however the device can't really
handle that).

Or, if the guest driver is capable to drop F_FREE_PAGE_HINT when
F_POISON is not declared, we can safely split the two features into
two patches in QEMU too.

Regards,

-- 
Peter Xu