Re: [Qemu-devel] [V2 PATCH] rtl8139: check the buffer availiability

[Michael S. Tsirkin]

On Mon, Oct 17, 2011 at 10:55:57AM +0800, Jason Wang wrote:
> Reduce spurious packet drops on RX ring empty when in c+ mode by verifying 
> that
> we have at least 1 buffer ahead of the time.
> 
> Change from v1:
> Fix style comments from Stefan.
> 
> Signed-off-by: Jason Wang <address@hidden>
> ---
>  hw/rtl8139.c |   44 ++++++++++++++++++++++++++++++--------------
>  1 files changed, 30 insertions(+), 14 deletions(-)
> 
> diff --git a/hw/rtl8139.c b/hw/rtl8139.c
> index 3753950..bcbc5e3 100644
> --- a/hw/rtl8139.c
> +++ b/hw/rtl8139.c
> @@ -84,6 +84,19 @@
>  #define VLAN_TCI_LEN 2
>  #define VLAN_HLEN (ETHER_TYPE_LEN + VLAN_TCI_LEN)
>  
> +/* w0 ownership flag */
> +#define CP_RX_OWN (1<<31)
> +/* w0 end of ring flag */
> +#define CP_RX_EOR (1<<30)
> +/* w0 bits 0...12 : buffer size */
> +#define CP_RX_BUFFER_SIZE_MASK ((1<<13) - 1)
> +/* w1 tag available flag */
> +#define CP_RX_TAVA (1<<16)
> +/* w1 bits 0...15 : VLAN tag */
> +#define CP_RX_VLAN_TAG_MASK ((1<<16) - 1)
> +/* w2 low  32bit of Rx buffer ptr */
> +/* w3 high 32bit of Rx buffer ptr */
> +
>  #if defined (DEBUG_RTL8139)
>  #  define DPRINTF(fmt, ...) \
>      do { fprintf(stderr, "RTL8139: " fmt, ## __VA_ARGS__); } while (0)
> @@ -805,6 +818,22 @@ static inline target_phys_addr_t rtl8139_addr64(uint32_t 
> low, uint32_t high)
>  #endif
>  }
>  
> +/* Verify that we have at least one available rx buffer */
> +static int rtl8139_cp_has_rxbuf(RTL8139State *s)
> +{
> +    uint32_t val, rxdw0;
> +    target_phys_addr_t cplus_rx_ring_desc = rtl8139_addr64(s->RxRingAddrLO,
> +                                                           s->RxRingAddrHI);
> +    cplus_rx_ring_desc += 16 * s->currCPlusRxDesc;
> +    cpu_physical_memory_read(cplus_rx_ring_desc, &val, 4);

Interesting. Please note that cpu_physical_memory_read is not
done atomically. Can guest be writing the value while
we read it? If yes we'll get a corrupted value here,
because CP_RX_OWN is the high bit so it is read last.
Correct?

I realize we have the same pattern in other places in this
device, probably a bug as well?


> +    rxdw0 = le32_to_cpu(val);
> +    if (rxdw0 & CP_RX_OWN) {
> +        return 1;
> +    } else {
> +        return 0;
> +    }

Do we need to check that buffer size is large enough
to include the packet like we do for non c+ mode?

> +}
> +
>  static int rtl8139_can_receive(VLANClientState *nc)
>  {
>      RTL8139State *s = DO_UPCAST(NICState, nc, nc)->opaque;
> @@ -819,7 +848,7 @@ static int rtl8139_can_receive(VLANClientState *nc)
>      if (rtl8139_cp_receiver_enabled(s)) {
>          /* ??? Flow control not implemented in c+ mode.
>             This is a hack to work around slirp deficiencies anyway.  */
> -        return 1;
> +        return rtl8139_cp_has_rxbuf(s);
>      } else {
>          avail = MOD2(s->RxBufferSize + s->RxBufPtr - s->RxBufAddr,
>                       s->RxBufferSize);
> @@ -965,19 +994,6 @@ static ssize_t rtl8139_do_receive(VLANClientState *nc, 
> const uint8_t *buf, size_
>  
>          /* begin C+ receiver mode */
>  
> -/* w0 ownership flag */
> -#define CP_RX_OWN (1<<31)
> -/* w0 end of ring flag */
> -#define CP_RX_EOR (1<<30)
> -/* w0 bits 0...12 : buffer size */
> -#define CP_RX_BUFFER_SIZE_MASK ((1<<13) - 1)
> -/* w1 tag available flag */
> -#define CP_RX_TAVA (1<<16)
> -/* w1 bits 0...15 : VLAN tag */
> -#define CP_RX_VLAN_TAG_MASK ((1<<16) - 1)
> -/* w2 low  32bit of Rx buffer ptr */
> -/* w3 high 32bit of Rx buffer ptr */
> -
>          int descriptor = s->currCPlusRxDesc;
>          target_phys_addr_t cplus_rx_ring_desc;
>