intel checksum offload

[Jack Vogel]

On Sun, Sep 18, 2011 at 7:48 PM, Arnaud Lacombe <lacombar at gmail.com> wrote:

> Hi,
>
> On Sun, Sep 18, 2011 at 10:01 PM, Luigi Rizzo <rizzo at iet.unipi.it> wrote:
> > On Sun, Sep 18, 2011 at 06:05:33PM -0400, Arnaud Lacombe wrote:
> >> Hi,
> >>
> >> On Sun, Sep 18, 2011 at 5:06 PM, Luigi Rizzo <rizzo at iet.unipi.it>
> wrote:
> >> > On Sun, Sep 18, 2011 at 03:19:46PM -0400, Arnaud Lacombe wrote:
> >> >> Hi,
> >> >>
> >> >> On Sat, Sep 17, 2011 at 4:32 PM, YongHyeon PYUN <pyunyh at gmail.com>
> wrote:
> >> >> > On Sat, Sep 17, 2011 at 11:57:10AM +0430, Hooman Fazaeli wrote:
> >> >> >> Hi list,
> >> >> >>
> >> >> >> The data sheet for intel 82576 advertises IP TX/RX checksum
> offload
> >> >> >> but the driver does not set CSUM_IP in ifp->if_hwassist. Does this
> mean that
> >> >> >> driver (and chip) do not support IP TX checksum offload or the
> support for
> >> >> >> TX is not yet included in the driver?
> >> > ...
> >> >> This is slightly off-topic, but still..
> >> >>
> >> >> FWIW, I'm not really impressed by what chips claim to support vs.
> what
> >> >> has been implemented in the driver. As per the product brief, the
> >> > ...
> >> >> [0]: the commit message say "performance was not good", but it is not
> >> >> the driver's developer to decide whether or not a feature is good or
> >> >> not. The developer's job is to implement the chip capabilities, and
> >> >> let it to the user to enable or disable the capabilities. At best,
> the
> >> >> developer can decide whether or not to enable the feature by default.
> >> >
> >> > actually, this is a perfect example where the developer has done the
> >> > right thing: implemented the feature, verified that performance is
> bad,
> >> > hence presumably removed support for the feature from the code (which
> also
> >> > means that the normal code path will run faster because there are no
> >> > run-time decisions to be made).
> >> >
> >> > "optional" features are often costly even when disabled.
> >> >
> >> I forgot to mention that in this case, the code full of
> >> EM_MULTIQUEUE's #ifdef and shared code is still fully compatible with
> >> the multiqueue's architecture. The only thing removed is a conditional
> >> and an assignation in the driver's attachment which was enabling the
> >> feature, ie. the cost you point out is still paid today, without any
> >> benefit.
> >
> > the above suggests that you have a wonderful opportunity: with just
> > a little bit of time and effort you should be able to complete/re-enable
> > the missing code, run tests that you believe significant (given
> > your statement below) and prove or disprove the comment about
> > performance.
> >
> Which I did about a week ago, to finally discover that the NIC only
> had only 3 MSI-X vectors configured in its EEPROM[0], and thus the
> MSI-X PCI capability field ends up also with being assigned with those
> 3 vectors. However, the  82574 datasheet clearly say that up-to 5
> vector can be configured, but I obviously did not find the magic trick
> to make it so. Maybe I'll find some time and try to reprogram the
> EEPROM. Beside that, it was clear that the old multiqueue did not
> support only 3 vector being available and thus fell back on MSI. It is
> not clear in jfv@'s comment whether he really tested multiqueue, or
> did he test the fall-back MSI mode.
>
> As the PCI spec is not public, I've not been able to find out from the
> few public datasheet how the PCI MSI-X capability field is first
> programmed. I'd assume that the BIOS is using the data in the NVM to
> program it at power up.
>
>  - Arnaud
>
> [0]: at least, the MSI_X_NUM field of the NVM at offset 0x1b is 2,
> thus 3 vectors.
>

I give answers to those who treat me with respect, I view them as
collaborators, we improve the drivers for everyone's benefit, rather
than jumping in to throw a critical remark here, a negative innuendo
there...

If you notice, the Linux driver did not enable multiqueue on the hardware
either, so do you think a whole department of software engineers backed
by the hardware engineers who designed the damn thing might have had
a reason?

IN FACT, as I have a bit more freedom with FreeBSD, I went ahead and
tried it for a while just because I could, implementing the code was not
difficult. Over time however that code proved to be a source of instability
and thus was disabled.

I have heard a rumor that the Linux crew may actually be trying a second
time to make it work, and that might give me cause to look at it again too,
but its not clear if I'll have time with other priorities.

Jack