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

[Alex Williamson <alex.williamson@redhat.com>]

On Mon, 2013-11-18 at 15:57 -0500, Vlad Yasevich wrote:
> 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.

But doesn't that valid bit mean that a mac update will start and end
with a write to the high order register?  So we're assuming:

a) write RA + 1 (invalidate)
b) write RA (write low)
c) write RA + 1 (write high + valid)

Without cd5be5829 the only change is that we don't store a new mac into
the monitor at b).  The mac stored in the monitor is still wrong from a)
until c).  So it's ever so slightly less broken without 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 :)

So the argument for reverting here is that it seems unlikely that the
dwords get written in the hi->lo order and we'd rather the monitor get
stuck with the wrong mac forever than it show the wrong mac address for
a tiny window for everybody else?  I think you say something about
sub-optimal here...

> 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.

Exactly, the code isn't perfect either way and this revert is just
replacing one sub-optimal solution for another.  So why do it?  Thanks,

Alex

> >> 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;
> >>>
> >>>
> >>>
> >>
> > 
> > 
> > 
>