netdev - Re: [PATCH bpf-next v4 05/17] xsk: Change the default frame size to 4096 and allow controlling it

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Message-ID: <20190613102936.2c8979ed@cakuba.netronome.com>
Date:   Thu, 13 Jun 2019 10:29:36 -0700
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     Maxim Mikityanskiy <maximmi@...lanox.com>
Cc:     Jesper Dangaard Brouer <brouer@...hat.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Björn Töpel 
        <bjorn.topel@...el.com>,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Saeed Mahameed <saeedm@...lanox.com>,
        Jonathan Lemon <bsd@...com>,
        Tariq Toukan <tariqt@...lanox.com>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Maciej Fijalkowski <maciejromanfijalkowski@...il.com>
Subject: Re: [PATCH bpf-next v4 05/17] xsk: Change the default frame size to
 4096 and allow controlling it

On Thu, 13 Jun 2019 14:01:39 +0000, Maxim Mikityanskiy wrote:
> On 2019-06-12 23:10, Jakub Kicinski wrote:
> > On Wed, 12 Jun 2019 15:56:43 +0000, Maxim Mikityanskiy wrote:  
> >> The typical XDP memory scheme is one packet per page. Change the AF_XDP
> >> frame size in libbpf to 4096, which is the page size on x86, to allow
> >> libbpf to be used with the drivers with the packet-per-page scheme.  
> > 
> > This is slightly surprising.  Why does the driver care about the bufsz?  
> 
> The classic XDP implementation supports only the packet-per-page scheme. 
> mlx5e implements this scheme, because it perfectly fits with xdp_return 
> and page pool APIs. AF_XDP relies on XDP, and even though AF_XDP doesn't 
> really allocate or release pages, it works on top of XDP, and XDP 
> implementation in mlx5e does allocate and release pages (in general 
> case) and works with the packet-per-page scheme.

Yes, okay, I get that.  But I still don't know what's the exact use you
have for AF_XDP buffers being 4k..  Could you point us in the code to
the place which relies on all buffers being 4k in any XDP scenario?

> > You're not supposed to so page operations on UMEM pages, anyway.
> > And the RX size filter should be configured according to MTU regardless
> > of XDP state.  
> 
> Yes, of course, MTU is taken into account.
> 
> > Can you explain?
> >   
> >> Add a command line option -f to xdpsock to allow to specify a custom
> >> frame size.
> >>
> >> Signed-off-by: Maxim Mikityanskiy <maximmi@...lanox.com>
> >> Reviewed-by: Tariq Toukan <tariqt@...lanox.com>
> >> Acked-by: Saeed Mahameed <saeedm@...lanox.com>