Re: [Qemu-devel] [PATCH] Make SLIRP Ethernet packets size to 64 bytes minimum

[Stefan Hajnoczi]

On Mon, Jun 27, 2011 at 5:12 PM, Fabien Chouteau <address@hidden> wrote:
> On 27/06/2011 17:39, Stefan Hajnoczi wrote:
>> On Mon, Jun 27, 2011 at 3:23 PM, Fabien Chouteau <address@hidden> wrote:
>>> On 27/06/2011 15:50, Stefan Hajnoczi wrote:
>>>> On Mon, Jun 27, 2011 at 1:41 PM, Fabien Chouteau <address@hidden> wrote:
>>>>>
>>>>> Signed-off-by: Fabien Chouteau <address@hidden>
>>>>> ---
>>>>>  slirp/slirp.c |    4 ++--
>>>>>  1 files changed, 2 insertions(+), 2 deletions(-)
>>>>
>>>> Any particular bug that this fixes?
>>>>
>>>> There have been 64 byte minimum padding patches to several emulated
>>>> NICs.  There has also been discussion about where the best place to do
>>>> this is.  Why is this patch necessary?
>>>>
>>>
>>> This patch is necessary because some NICs are configured to drop short 
>>> frames,
>>> therefore the OS will not receive some of the packets generated by Qemu.
>>>
>>> There's a first patch to fix this issue:
>>> http://git.savannah.gnu.org/cgit/qemu.git/commit/?id=dbf3c4b4baceb91eb64d09f787cbe92d65188813
>>>
>>> My patch fixes two other sources of short frames.
>>
>> Thanks for the explanation.  I stepped back from the discussion on
>> where the right place to fix this is last time around.  Now I'm
>> wondering why do anything in slirp when the other sources (tap, ...)
>> aren't padding to 64 bytes?
>
> I think that packets generated by Qemu must follow RFC. For other sources, 
> qemu
> should keep original size when possible and put padding otherwise.

slirp interfaces to net.h which does not emulate Ethernet.  For
example, it doesn't carry the Frame Check Sequence (FCS) as per the
Ethernet standard.  NICs that want to report FCS to the guest
calculate it themselves, just like they do with 64-byte padding.  So I
find it weird to say we should honor Ethernet when the transport we
are using is not Ethernet.

This patch doesn't hurt but we'd be just as well off without it.

Did you do this to fix a bug?  If so, then something else in QEMU
needs to be fixed, not slirp.

Stefan