Re: [Qemu-devel] [PATCH] virtio-spec: split virtio-net device status filed into ro and rw byte

[Jason Wang <jasowang@redhat.com>]

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@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 ...
>

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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