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Re: [Qemu-devel] [PATCH v7 4/5] virtio-balloon: VIRTIO_BALLOON_F_FREE_PA
From: |
Michael S. Tsirkin |
Subject: |
Re: [Qemu-devel] [PATCH v7 4/5] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT |
Date: |
Fri, 8 Jun 2018 05:49:26 +0300 |
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
- Re: [Qemu-devel] [PATCH v7 4/5] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT, (continued)
- Re: [Qemu-devel] [PATCH v7 4/5] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT, Peter Xu, 2018/06/06
- Re: [Qemu-devel] [PATCH v7 4/5] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT, Wei Wang, 2018/06/06
- Re: [Qemu-devel] [PATCH v7 4/5] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT, Peter Xu, 2018/06/06
- Re: [Qemu-devel] [PATCH v7 4/5] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT, Wei Wang, 2018/06/07
- Re: [Qemu-devel] [PATCH v7 4/5] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT, Peter Xu, 2018/06/07
- Re: [Qemu-devel] [PATCH v7 4/5] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT, Wei Wang, 2018/06/07
- Re: [Qemu-devel] [PATCH v7 4/5] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT, Peter Xu, 2018/06/07
- Re: [Qemu-devel] [PATCH v7 4/5] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT, Michael S. Tsirkin, 2018/06/07
- Re: [Qemu-devel] [PATCH v7 4/5] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT, Peter Xu, 2018/06/07
- Re: [Qemu-devel] [PATCH v7 4/5] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT,
Michael S. Tsirkin <=
- Re: [Qemu-devel] [PATCH v7 4/5] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT, Peter Xu, 2018/06/07
- Re: [Qemu-devel] [PATCH v7 4/5] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT, Peter Xu, 2018/06/07