From patchwork Tue Nov 24 21:27:18 2009
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Subject: Re: [PATCH] Fix TAP networking on host kernels without IFF_VNET_HDR
 support
Date: Tue, 24 Nov 2009 11:27:18 -0000
From: Pierre Riteau <Pierre.Riteau@irisa.fr>
X-Patchwork-Id: 39240
Message-Id: <14A83280-2A2D-4DCC-BD75-5DB6B43883EB@irisa.fr>
To: Mark McLoughlin <markmc@redhat.com>
Cc: qemu-devel@nongnu.org

On 24 nov. 2009, at 12:22, Mark McLoughlin wrote:

> On Tue, 2009-11-24 at 12:17 +0100, Pierre Riteau wrote:
>> On 24 nov. 2009, at 11:28, Mark McLoughlin wrote:
>> 
>>> On Tue, 2009-11-24 at 10:06 +0100, Pierre Riteau wrote:
>>>> vnet_hdr is initialized at 1 by default. We need to reset it to 0 if
>>>> the kernel doesn't support IFF_VNET_HDR.
>>>> 
>>>> Signed-off-by: Pierre Riteau <Pierre.Riteau@irisa.fr>
>>> 
>>> Thanks Pierre, I see why this is needed now
>>> 
>>> Acked-by: Mark McLoughlin <markmc@redhat.com>
>>> 
>>> Cheers,
>>> Mark.
>> 
>> 
>> Thanks for your rapid answer!
>> 
>> BTW, every time I run qemu I see this error message:
>> 
>>    TUNSETOFFLOAD ioctl() failed: Invalid argument
>> 
>> It is caused by the piece of code at the end of net/tap-linux.c:
>> 
>>    if (ioctl(fd, TUNSETOFFLOAD, offload) != 0) {
>>        offload &= ~TUN_F_UFO;
>>        if (ioctl(fd, TUNSETOFFLOAD, offload) != 0) {
>>            fprintf(stderr, "TUNSETOFFLOAD ioctl() failed: %s\n",
>>                    strerror(errno));
>>        }
>>    }
>> 
>> Isn't there a way to detect whether the kernel supports the
>> TUNSETOFFLOAD ioctl at all?
> 
> The kernel will set errno to EINVAL if TUNSETOFFLOAD isn't supported, so
> we could just ignore that case:
> 
>     if (ioctl(fd, TUNSETOFFLOAD, offload) != 0) {
>          offload &= ~TUN_F_UFO;
>          if (ioctl(fd, TUNSETOFFLOAD, offload) != 0 && errno != EINVAL) {
>              fprintf(stderr, "TUNSETOFFLOAD ioctl() failed: %s\n",
>                      strerror(errno));
>          }
>      }
> 
> The only concern is that we'll also miss out on an error message if
> EINVAL is set for another reason. Currently, the only other reason if we
> pass a offload flag not supported by the kernel, but that should never
> happen.
> 
> Feel free to send a patch with that change and I'll ack it
> 
> Thanks,
> Mark.
> 


Couldn't we probe the kernel with a 0 offload value to check if it supports TUNSETOFFLOAD?
I tried the following and it works on my 2.6.26 Debian kernel.
What do you think? I will send a proper patch if you agree.


diff --git a/net/tap-linux.c b/net/tap-linux.c
index 0f621a2..e038e1a 100644
--- a/net/tap-linux.c
+++ b/net/tap-linux.c
@@ -129,6 +129,11 @@ void tap_fd_set_offload(int fd, int csum, int tso4,
 {
     unsigned int offload = 0;
 
+    /* Check if our kernel supports TUNSETOFFLOAD */
+    if (ioctl(fd, TUNSETOFFLOAD, 0) != 0 && errno == EINVAL) {
+        return;
+    }
+
     if (csum) {
         offload |= TUN_F_CSUM;
         if (tso4)