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Message-ID: <4F66A364.2080600@redhat.com> Date: Mon, 19 Mar 2012 11:09:24 +0800 From: Jason Wang <jasowang@...hat.com> To: "Michael S. Tsirkin" <mst@...hat.com> CC: qemu-devel@...gnu.org, rusty@...tcorp.com.au, virtualization@...ts.linux-foundation.org, linux-kernel@...r.kernel.org Subject: Re: [PATCH] virtio-spec: split virtio-net device status filed into ro and rw byte On 03/18/2012 08:22 PM, Michael S. Tsirkin wrote: > 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@...hat.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 ... > Didn't follow, do you mean to make VIRITO_NET_S_ANNOUNCE bit clear on read? Looks like this can prevent the race and keep what currently we have. > >> --- >> 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 > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@...r.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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