On 08/31/2015 10:24 PM, Vlad Yasevich wrote: > On 08/31/2015 05:59 AM, Jason Wang wrote: >> >> On 08/28/2015 10:06 PM, Vladislav Yasevich wrote: >>> In standard operation mode, when the receive ring buffer >>> is full, the buffer actually appears empty to the driver since >>> the RxBufAddr (the location we wirte new data to) and RxBufPtr >>> (the location guest would stat reading from) are the same. >>> As a result, the call to rtl8139_RxBufferEmpty ends up >>> returning true indicating that the receive buffer is empty. >>> This would result in the next packet overwriting the recevie buffer >>> again and stalling receive operations. >>> >>> This patch tracks the number of unread bytes in the rxbuffer >>> using an unused C+ register. On every read and write, the >>> number is adjsted and the special case of a full buffer is also >>> trapped. >>> >>> The C+ register trick is used to simplify migration and not require >>> a new machine type. This register is not used in regular mode >>> and C+ mode doesn't have the same issue. >>> >>> Signed-off-by: Vladislav Yasevich <vyase...@redhat.com> >>> --- >>> hw/net/rtl8139.c | 45 +++++++++++++++++++++++++++++++++++++++++---- >>> 1 file changed, 41 insertions(+), 4 deletions(-) >> I'm not sure this can happen. For example, looks like the following >> check in rtl8139_do_receive(): >> >> if (avail != 0 && size + 8 >= avail) >> { >> >> can guarantee there's no overwriting? >> > The problem is the calculation of avail. When the buffer is full, > avail will be the the size of the receive buffer. So the test > above will be false because the driver thinks there is actually > enough room. > > With his patch, 'avail' will be calculated to 0. > > -vlad >
If believe the condition size + 8 >= avail can guarantee that the buffer won't be full (if we allow size + 8 == avail, buffer will be full)? So avail == 0 means the buffer is empty. Or is there anything I miss?