netdev - Re: [PATCH bpf-next] libbpf: add xsk_umem__adjust

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Message-ID: <2c14280f-a361-7d36-ffb0-02c2046fa3a1@fb.com>
Date:   Fri, 13 Sep 2019 22:49:55 +0000
From:   Yonghong Song <yhs@...com>
To:     Kevin Laatz <kevin.laatz@...el.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "ast@...nel.org" <ast@...nel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        "bjorn.topel@...el.com" <bjorn.topel@...el.com>,
        "magnus.karlsson@...el.com" <magnus.karlsson@...el.com>,
        "jonathan.lemon@...il.com" <jonathan.lemon@...il.com>
CC:     "bruce.richardson@...el.com" <bruce.richardson@...el.com>,
        "ciara.loftus@...el.com" <ciara.loftus@...el.com>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf-next] libbpf: add xsk_umem__adjust_offset



On 9/12/19 8:28 AM, Kevin Laatz wrote:
> Currently, xsk_umem_adjust_offset exists as a kernel internal function.
> This patch adds xsk_umem__adjust_offset to libbpf so that it can be used
> from userspace. This will take the responsibility of properly storing the
> offset away from the application, making it less error prone.
> 
> Since xsk_umem__adjust_offset is called on a per-packet basis, we need to
> inline the function to avoid any performance regressions.  In order to
> inline xsk_umem__adjust_offset, we need to add it to xsk.h. Unfortunately
> this means that we can't dereference the xsk_umem_config struct directly
> since it is defined only in xsk.c. We therefore add an extra API to return
> the flags field to the user from the structure, and have the inline
> function use this flags field directly.

Could you add an example (either in samples/bpf or in 
tools/testing/selftests/bpf) to use xsk_umem__adjust_offset()? This will 
make it
easier to check whether the interface is appropriate or not and
what is the performance implication as you stated in the above.

> 
> Signed-off-by: Kevin Laatz <kevin.laatz@...el.com>
> ---
>   tools/lib/bpf/libbpf.map |  1 +
>   tools/lib/bpf/xsk.c      |  5 +++++
>   tools/lib/bpf/xsk.h      | 14 ++++++++++++++
>   3 files changed, 20 insertions(+)
> 
> diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
> index d04c7cb623ed..760350c9b81c 100644
> --- a/tools/lib/bpf/libbpf.map
> +++ b/tools/lib/bpf/libbpf.map
> @@ -189,4 +189,5 @@ LIBBPF_0.0.4 {
>   LIBBPF_0.0.5 {
>   	global:
>   		bpf_btf_get_next_id;
> +		xsk_umem__get_flags;
>   } LIBBPF_0.0.4;
> diff --git a/tools/lib/bpf/xsk.c b/tools/lib/bpf/xsk.c
> index 842c4fd55859..a4250a721ea6 100644
> --- a/tools/lib/bpf/xsk.c
> +++ b/tools/lib/bpf/xsk.c
> @@ -84,6 +84,11 @@ int xsk_socket__fd(const struct xsk_socket *xsk)
>   	return xsk ? xsk->fd : -EINVAL;
>   }
>   
> +__u32 xsk_umem__get_flags(struct xsk_umem *umem)
> +{
> +	return umem->config.flags;
> +}
> +
>   static bool xsk_page_aligned(void *buffer)
>   {
>   	unsigned long addr = (unsigned long)buffer;
> diff --git a/tools/lib/bpf/xsk.h b/tools/lib/bpf/xsk.h
> index 584f6820a639..bf782facb274 100644
> --- a/tools/lib/bpf/xsk.h
> +++ b/tools/lib/bpf/xsk.h
> @@ -183,8 +183,22 @@ static inline __u64 xsk_umem__add_offset_to_addr(__u64 addr)
>   	return xsk_umem__extract_addr(addr) + xsk_umem__extract_offset(addr);
>   }
>   
> +/* Handle the offset appropriately depending on aligned or unaligned mode.
> + * For unaligned mode, we store the offset in the upper 16-bits of the address.
> + * For aligned mode, we simply add the offset to the address.
> + */
> +static inline __u64 xsk_umem__adjust_offset(__u32 umem_flags, __u64 addr,
> +					    __u64 offset)
> +{
> +	if (umem_flags & XDP_UMEM_UNALIGNED_CHUNK_FLAG)
> +		return addr + (offset << XSK_UNALIGNED_BUF_OFFSET_SHIFT);
> +	else
> +		return addr + offset;
> +}
> +
>   LIBBPF_API int xsk_umem__fd(const struct xsk_umem *umem);
>   LIBBPF_API int xsk_socket__fd(const struct xsk_socket *xsk);
> +LIBBPF_API __u32 xsk_umem__get_flags(struct xsk_umem *umem);
>   
>   #define XSK_RING_CONS__DEFAULT_NUM_DESCS      2048
>   #define XSK_RING_PROD__DEFAULT_NUM_DESCS      2048
>