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    SubjectRe: [PATCH] virtio-spec: split virtio-net device status filed into ro and rw byte
    On Fri, Mar 16, 2012 at 11:20:26PM +0800, Jason Wang wrote:
    > This patch splits the device status field of virtio-net into ro and rw
    > byte. This would simplify the implementation of both host and guest
    > and make the layout more clean. As VIRTIO_NET_S_ANNOUNCE is a rw bit,
    > it was moved to bit 8 (0x100).
    >
    > btw. looks like there's no implementation that depends on
    > VIRTIO_NET_S_ANNOUNCE, so the move is safe.
    >
    > Signed-off-by: Jason Wang <jasowang@redhat.com>

    Hmm, I know I proposed this myself, and I thought it will
    prevent problems if we will add more rw bits,
    but I missed the following race:

    host writes VIRTIO_NET_S_ANNOUNCE = 1, interrupt

    guest reads VIRTIO_NET_S_ANNOUNCE = 1

    host writes VIRTIO_NET_S_SOME_NEW_FIELD = 1, interrupt

    guest writes VIRTIO_NET_S_ANNOUNCE = 0
    VIRTIO_NET_S_SOME_NEW_FIELD is overwritten

    guest reads VIRTIO_NET_S_SOME_NEW_FIELD = 0

    How about making the new bit write 1 to clear?
    If we do, we can keep it where it is currently ...



    > ---
    > virtio-0.9.4.lyx | 20 +++++++++++++++++++-
    > 1 files changed, 19 insertions(+), 1 deletions(-)
    >
    > diff --git a/virtio-0.9.4.lyx b/virtio-0.9.4.lyx
    > index 6c7bab1..ef3951c 100644
    > --- a/virtio-0.9.4.lyx
    > +++ b/virtio-0.9.4.lyx
    > @@ -58,6 +58,7 @@
    > \html_be_strict false
    > \author -608949062 "Rusty Russell,,,"
    > \author 1531152142 "pbonzini"
    > +\author 2090695081 "Jason"
    > \end_header
    >
    > \begin_body
    > @@ -4012,8 +4013,19 @@ configuration
    > layout Two configuration fields are currently defined.
    > The mac address field always exists (though is only valid if VIRTIO_NET_F_MAC
    > is set), and the status field only exists if VIRTIO_NET_F_STATUS is set.
    > +
    > +\change_inserted 2090695081 1331907586
    > + The low byte of status field is read-only, guest write to this byte would
    > + be ignored.
    > + Currently only one bit is defined for this byte: VIRTIO_NET_S_LINK_UP.
    > + The high byte of status field is read-writable.
    > + Currently only one bit is defined for this byte: VIRTIO_NET_S_ANNOUNCE.
    > +
    > +\change_deleted 2090695081 1331907489
    > Two bits are currently defined for the status field: VIRTIO_NET_S_LINK_UP
    > and VIRTIO_NET_S_ANNOUNCE.
    > +
    > +\change_unchanged
    >
    > \begin_inset listings
    > inline false
    > @@ -4026,7 +4038,13 @@ status open
    >
    > \begin_layout Plain Layout
    >
    > -#define VIRTIO_NET_S_ANNOUNCE 2
    > +#define VIRTIO_NET_S_ANNOUNCE
    > +\change_inserted 2090695081 1331907493
    > +0x100
    > +\change_deleted 2090695081 1331907491
    > +2
    > +\change_unchanged
    > +
    > \end_layout
    >
    > \begin_layout Plain Layout


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