Re: [PATCH v3 01/11] tap: Remove tap_probe_vnet_hdr_len()

[Akihiko Odaki]

On 2023/10/13 23:32, Michael S. Tsirkin wrote:
> On Fri, Oct 13, 2023 at 11:22:10PM +0900, Akihiko Odaki wrote:
> > On 2023/10/13 23:17, Michael S. Tsirkin wrote:
> > > On Fri, Oct 13, 2023 at 02:26:03PM +0900, Akihiko Odaki wrote:
> > > > On 2023/10/13 14:00, Jason Wang wrote:
> > > > > On Fri, Oct 13, 2023 at 12:14 PM Akihiko Odaki <akihiko.odaki@daynix.com> wrote:
> > > > > > On 2023/10/13 10:38, Jason Wang wrote:
> > > > > > > On Wed, Oct 11, 2023 at 11:40 PM Akihiko Odaki <akihiko.odaki@daynix.com> wrote:
> > > > > > > > It was necessary since an Linux older than 2.6.35 may implement the
> > > > > > > > virtio-net header but may not allow to change its length. Remove it
> > > > > > > > since such an old Linux is no longer supported.
> > > > > > >
> > > > > > > Where can I see this agreement?
> > > > > >
> > > > > > docs/about/build-platforms.rst says:
> > > > > >     > The project aims to support the most recent major version at all times
> > > > > >     > for up to five years after its initial release. Support for the
> > > > > >     > previous major version will be dropped 2 years after the new major
> > > > > >     > version is released or when the vendor itself drops support, whichever
> > > > > >     > comes first. In this context, third-party efforts to extend the
> > > > > >     > lifetime of a distro are not considered, even when they are endorsed
> > > > > >     > by the vendor (eg. Debian LTS); the same is true of repositories that
> > > > > >     > contain packages backported from later releases (e.g. Debian
> > > > > >     > backports). Within each major release, only the most recent minor
> > > > > >     > release is considered.
> > > > > >     >
> > > > > >     > For the purposes of identifying supported software versions available
> > > > > >     > on Linux, the project will look at CentOS, Debian, Fedora, openSUSE,
> > > > > >     > RHEL, SLES and Ubuntu LTS. Other distros will be assumed to ship
> > > > > >     > similar software versions.
> > > > >
> > > > > Well it also says:
> > > > >
> > > > > """
> > > > > If a platform is not listed here, it does not imply that QEMU won't
> > > > > work. If an unlisted platform has comparable software versions to a
> > > > > listed platform, there is every expectation that it will work.
> > > > > """
> > > > >
> > > > > A lot of downstream have customized build scripts.
> > > >
> > > > Still Linux versions older than 2.6.35 do not look like "comparable software
> > > > versions to a listed platform" in my opinion.
> > >
> > >
> > > This is fine - I would be ok to replace support with an error message
> > > and failure. Not checking that a capability is supported however
> > > isn't a good idea. And once we do - do we still gain anything by
> > > not working around that?
> >
> > tap does still check if setting the header length succeeds so it should be
> > fine.
>
> It asserts though doesn't it? Hardly user friendly ...

It prints an error message so the user should be able to figure out 
what's missing:
        fprintf(stderr, "TUNSETVNETHDRSZ ioctl() failed: %s. Exiting.\n",
                strerror(errno));