Re: [Qemu-devel] [PATCH for-1.7] Revert "e1000/rtl8139: update HMP NIC when every bit is written"

[Vlad Yasevich <vyasevic@redhat.com>]

On 11/18/2013 03:33 PM, Alex Williamson wrote:
> On Mon, 2013-11-18 at 15:09 -0500, Vlad Yasevich wrote:
>> On 11/18/2013 02:58 PM, Alex Williamson wrote:
>>> On Mon, 2013-11-18 at 21:47 +0200, Michael S. Tsirkin wrote:
>>>> This reverts commit cd5be5829c1ce87aa6b3a7806524fac07ac9a757.
>>>> Digging into hardware specs shows this does not
>>>> actually make QEMU behave more like hardware.
>>>> Let's stick to the tried heuristic for 1.7 and
>>>> possibly revisit for 1.8.
>>>
>>> If this is broken, then so are these:
>>>
>>> 23c37c37f0280761072c23bf67d3a4f3c0ff25aa
>>> 7c36507c2b8776266f50c5e2739bd18279953b93
>>
>> These aren't really broken.  They just assume that the high order
>> writes will happen after the low order writes.
>>
>> In the case of e1000, this is a little more then an assumption
>> (particularly in the case of nic initilization).
> 
> But AIUI there's also a valid bit in that high order byte on e1000, so
> reverting cd5be582 means we stuff a new mac into qemu less often, but
> it's still only accurate some of the time.

Yes, there is a slight issue with validity of mac at the time of
processing packets.  I have an outstanding question on the Intel
list about this behavior with real HW.  But, with e1000, the validity
bit provides a much higher guarantee that a guest that will be
setting the mac address will write the high register second to
guarantee that when the valid bit is written, the mac is fully
valid.  As a result we don't really need the e1000 part of the
cd5be5829.


> 
>> In the case of RTL nic, it is just an  assumption, but it hasn't
>> been shown faulty yet.  We do plan to address this a bit more
>> thoroughly.
> 
> So how is RTL less broken without cd5be582?  AIUI the valid bit is off
> in a separate register on RTL, so we have no guarantee about order of
> updating the mac.  Without cd5be582 the info in the monitor may be
> permanently broken if the guest uses a write order other than what we
> assume.
> 

This one is actually not as bad either.  RTL spec requires that
receive register writes happen as 32 bit word writes.  This is
what linux and bsd drivers do, so from driver perspective, the
issue is the same.  What our emulation layer does is turn these
32 bit writes into 4 8-bit writes.  This is likely due to some
very broken and very old drivers, but I am not sure.

So, the information in the monitor will be broken if the guest
does: write_hi(); write_lo();  A part of me would really like
to see a guest that does this :)

The current code isn't perfect either.  It still has a potential
to show the wrong mac address in the monitor.  I doesn't make
a lot of sense to me to replace one sub-optimal solution
with another sub-optimal solution, especially since no-one
complained.

-vlad

>> The patch that was applied was controversial and more then 1 person
>> expressed reservations.
> 
> Understood, but it also raised and addressed a shortcoming in the
> previous patches.  If cd5be582 was controversial because the monitor was
> getting updated with incorrect mac addresses then this simple revert
> doesn't solve that problem.  Thanks,
> 
> Alex
> 
>>>
>>> None of these change the behavior of hardware, they only change when the
>>> monitor gets told about mac address changes.  I'd suggest either add the
>>> emulation described in each spec or revert all of them.  A partial
>>> revert is just noise.  Thanks,
>>>
>>> Alex
>>>
>>>>
>>>> Reported-by: Vlad Yasevich <vyasevic@redhat.com>
>>>> Cc: Amos Kong <akong@redhat.com>
>>>> Cc: Alex Williamson <alex.williamson@redhat.com>
>>>> ---
>>>>  hw/net/e1000.c   | 2 +-
>>>>  hw/net/rtl8139.c | 5 ++++-
>>>>  2 files changed, 5 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/hw/net/e1000.c b/hw/net/e1000.c
>>>> index ae63591..8387443 100644
>>>> --- a/hw/net/e1000.c
>>>> +++ b/hw/net/e1000.c
>>>> @@ -1106,7 +1106,7 @@ mac_writereg(E1000State *s, int index, uint32_t val)
>>>>  
>>>>      s->mac_reg[index] = val;
>>>>  
>>>> -    if (index == RA || index == RA + 1) {
>>>> +    if (index == RA + 1) {
>>>>          macaddr[0] = cpu_to_le32(s->mac_reg[RA]);
>>>>          macaddr[1] = cpu_to_le32(s->mac_reg[RA + 1]);
>>>>          qemu_format_nic_info_str(qemu_get_queue(s->nic), (uint8_t *)macaddr);
>>>> diff --git a/hw/net/rtl8139.c b/hw/net/rtl8139.c
>>>> index 7f2b4db..5329f44 100644
>>>> --- a/hw/net/rtl8139.c
>>>> +++ b/hw/net/rtl8139.c
>>>> @@ -2741,7 +2741,10 @@ static void rtl8139_io_writeb(void *opaque, uint8_t addr, uint32_t val)
>>>>  
>>>>      switch (addr)
>>>>      {
>>>> -        case MAC0 ... MAC0+5:
>>>> +        case MAC0 ... MAC0+4:
>>>> +            s->phys[addr - MAC0] = val;
>>>> +            break;
>>>> +        case MAC0+5:
>>>>              s->phys[addr - MAC0] = val;
>>>>              qemu_format_nic_info_str(qemu_get_queue(s->nic), s->phys);
>>>>              break;
>>>
>>>
>>>
>>
> 
> 
>