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

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Jun 08, 2018 at 10:34:25AM +0800, Peter Xu wrote:
> 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).

It seems to handle it fine, it just ignores the hints.

> 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