netdev - Re: [PATCH bpf-next v2 1/2] net: xdp: refactor XDP_QUERY_PROG{,

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Date:   Mon, 3 Jun 2019 10:03:21 -0700
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     Björn Töpel <bjorn.topel@...il.com>
Cc:     Saeed Mahameed <saeedm@...lanox.com>,
        "toke@...hat.com" <toke@...hat.com>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        "ast@...nel.org" <ast@...nel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "magnus.karlsson@...el.com" <magnus.karlsson@...el.com>,
        "bjorn.topel@...el.com" <bjorn.topel@...el.com>,
        "brouer@...hat.com" <brouer@...hat.com>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf-next v2 1/2] net: xdp: refactor XDP_QUERY_PROG{,_HW}
 to netdev

On Mon, 3 Jun 2019 11:04:36 +0200, Björn Töpel wrote:
> On Sat, 1 Jun 2019 at 21:57, Jakub Kicinski
> <jakub.kicinski@...ronome.com> wrote:
> >
> > On Fri, 31 May 2019 19:18:17 +0000, Saeed Mahameed wrote:  
> > > > +   if (!bpf_op || flags & XDP_FLAGS_SKB_MODE)
> > > > +           mode = XDP_FLAGS_SKB_MODE;
> > > > +
> > > > +   curr_mode = dev_xdp_current_mode(dev);
> > > > +
> > > > +   if (!offload && curr_mode && (mode ^ curr_mode) &
> > > > +       (XDP_FLAGS_DRV_MODE | XDP_FLAGS_SKB_MODE)) {  
> > >
> > > if i am reading this correctly this is equivalent to :
> > >
> > > if (!offload && (curre_mode != mode))
> > > offlad is false then curr_mode and mode must be DRV or GENERIC ..  
> >
> > Naw, if curr_mode is not set, i.e. nothing installed now, we don't care
> > about the diff.
> >  
> > > better if you keep bitwise operations for actual bitmasks, mode and
> > > curr_mode are not bitmask, they can hold one value each .. according to
> > > your logic..  
> >
> > Well, they hold one bit each, whether one bit is a bitmap perhaps is
> > disputable? :)
> >
> > I think the logic is fine.
> >  
> 
> Hmm, but changing to:
> 
>        if (!offload && curr_mode && mode != curr_mode)
> 
> is equal, and to Saeed's point, clearer. I'll go that route in a v3.

Sorry, you're right, the flags get mangled before they get here, so
yeah, this condition should work.  Confusingly.

> > What happened to my request to move the change in behaviour for
> > disabling to a separate patch, tho, Bjorn? :)  
> 
> Actually, I left that out completely. This patch doesn't change the
> behavior. After I realized how the flags *should* be used, I don't
> think my v1 change makes sense anymore. My v1 patch was to give an
> error if you tried to disable, say generic if drv was enabled via
> "auto detect/no flags". But this is catched by looking at the flags.
> 
> What I did, however, was moving the flags check into change_fd so that
> the driver doesn't have to do the check. E.g. the Intel drivers didn't
> do correct checking of flags.

Ugh.  Could you please rewrite the conditions to make the fd >= check
consistently the outside if?  Also could you add extack to this:

+	if (!offload && dev_xdp_query(dev, mode) &&
+	    !xdp_prog_flags_ok(dev->xdp_flags, flags, extack))
+		return -EBUSY;

It's unclear what it's doing.