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@xxxxxxxxxx>
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
--
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