Re: [PATCH v7 4/5] qmp: Added new command to retrieve eBPF blob.

[Akihiko Odaki <akihiko.odaki@gmail.com>]

On 2023/08/31 15:51, Andrew Melnychenko wrote:
> Now, the binary objects may be retrieved by id.
> It would require for future qmp commands that may require specific
> eBPF blob.
> 
> Added command "request-ebpf". This command returns
> eBPF program encoded base64. The program taken from the
> skeleton and essentially is an ELF object that can be
> loaded in the future with libbpf.
> 
> The reason to use the command to provide the eBPF object
> instead of a separate artifact was to avoid issues related
> to finding the eBPF itself. eBPF object is an ELF binary
> that contains the eBPF program and eBPF map description(BTF).
> Overall, eBPF object should contain the program and enough
> metadata to create/load eBPF with libbpf. As the eBPF
> maps/program should correspond to QEMU, the eBPF can't
> be used from different QEMU build.
> 
> The first solution was a helper that comes with QEMU
> and loads appropriate eBPF objects. And the issue is
> to find a proper helper if the system has several
> different QEMUs installed and/or built from the source,
> which helpers may not be compatible.
> 
> Another issue is QEMU updating while there is a running
> QEMU instance. With an updated helper, it may not be
> possible to hotplug virtio-net device to the already
> running QEMU. Overall, requesting the eBPF object from
> QEMU itself solves possible failures with acceptable effort.
> 
> Links:
> [PATCH 3/5] qmp: Added the helper stamp check.
> https://lore.kernel.org/all/20230219162100.174318-4-andrew@daynix.com/
> 
> Signed-off-by: Andrew Melnychenko <andrew@daynix.com>
> ---
>   ebpf/ebpf.c           | 70 +++++++++++++++++++++++++++++++++++++++++++
>   ebpf/ebpf.h           | 31 +++++++++++++++++++
>   ebpf/ebpf_rss.c       |  6 ++++
>   ebpf/meson.build      |  2 +-
>   qapi/ebpf.json        | 66 ++++++++++++++++++++++++++++++++++++++++
>   qapi/meson.build      |  1 +
>   qapi/qapi-schema.json |  1 +
>   7 files changed, 176 insertions(+), 1 deletion(-)
>   create mode 100644 ebpf/ebpf.c
>   create mode 100644 ebpf/ebpf.h
>   create mode 100644 qapi/ebpf.json
> 
> diff --git a/ebpf/ebpf.c b/ebpf/ebpf.c
> new file mode 100644
> index 0000000000..ea97c0403e
> --- /dev/null
> +++ b/ebpf/ebpf.c
> @@ -0,0 +1,70 @@
> +/*
> + * QEMU eBPF binary declaration routine.
> + *
> + * Developed by Daynix Computing LTD (http://www.daynix.com)
> + *
> + * Authors:
> + *  Andrew Melnychenko <andrew@daynix.com>
> + *
> + * This work is licensed under the terms of the GNU GPL, version 2 or
> + * later.  See the COPYING file in the top-level directory.
> + */

Use SPDX License Identifier. I had been asked the same thing but 
unfortunately it's not documented in docs/devel/style.rst.

> +
> +#include "qemu/osdep.h"
> +#include "qemu/queue.h"
> +#include "qapi/error.h"
> +#include "qapi/qapi-commands-ebpf.h"
> +#include "ebpf/ebpf.h"
> +
> +struct ElfBinaryDataEntry {
> +    int id;
> +    const void *data;
> +    size_t datalen;
> +
> +    QSLIST_ENTRY(ElfBinaryDataEntry) node;
> +};

Add typedef. See docs/devel/style.rst.

> +
> +static QSLIST_HEAD(, ElfBinaryDataEntry) ebpf_elf_obj_list =
> +                                            QSLIST_HEAD_INITIALIZER();

Why don't you use GHashTable?

> +
> +void ebpf_register_binary_data(int id, const void *data, size_t datalen)
> +{
> +    struct ElfBinaryDataEntry *dataentry = NULL;
> +
> +    dataentry = g_new0(struct ElfBinaryDataEntry, 1);
> +    dataentry->data = data;
> +    dataentry->datalen = datalen;
> +    dataentry->id = id;
> +
> +    QSLIST_INSERT_HEAD(&ebpf_elf_obj_list, dataentry, node);
> +}
> +
> +const void *ebpf_find_binary_by_id(int id, size_t *sz, Error **errp)
> +{
> +    struct ElfBinaryDataEntry *it = NULL;
> +    QSLIST_FOREACH(it, &ebpf_elf_obj_list, node) {
> +        if (id == it->id) {
> +            *sz = it->datalen;
> +            return it->data;
> +        }
> +    }
> +
> +    error_setg(errp, "can't find eBPF object with id: %d", id);
> +
> +    return NULL;
> +}
> +
> +EbpfObject *qmp_request_ebpf(EbpfProgramID id, Error **errp)
> +{
> +    EbpfObject *ret = NULL;
> +    size_t size = 0;
> +    const void *data = ebpf_find_binary_by_id(id, &size, errp);
> +    if (!data) {
> +        return NULL;
> +    }
> +
> +    ret = g_new0(EbpfObject, 1);
> +    ret->object = g_base64_encode(data, size);
> +
> +    return ret;
> +}
> diff --git a/ebpf/ebpf.h b/ebpf/ebpf.h
> new file mode 100644
> index 0000000000..b6266b28b8
> --- /dev/null
> +++ b/ebpf/ebpf.h
> @@ -0,0 +1,31 @@
> +/*
> + * QEMU eBPF binary declaration routine.
> + *
> + * Developed by Daynix Computing LTD (http://www.daynix.com)
> + *
> + * Authors:
> + *  Andrew Melnychenko <andrew@daynix.com>
> + *
> + * This work is licensed under the terms of the GNU GPL, version 2 or
> + * later.  See the COPYING file in the top-level directory.
> + */
> +
> +#ifndef EBPF_H
> +#define EBPF_H
> +
> +struct Error;

Why do you need this?

> +
> +void ebpf_register_binary_data(int id, const void *data,
> +                               size_t datalen);
> +const void *ebpf_find_binary_by_id(int id, size_t *sz,
> +                                   struct Error **errp);
> +
> +#define ebpf_binary_init(id, fn)                                           \
> +static void __attribute__((constructor)) ebpf_binary_init_ ## fn(void)     \
> +{                                                                          \
> +    size_t datalen = 0;                                                    \
> +    const void *data = fn(&datalen);                                       \
> +    ebpf_register_binary_data(id, data, datalen);                          \
> +}
> +
> +#endif /* EBPF_H */
> diff --git a/ebpf/ebpf_rss.c b/ebpf/ebpf_rss.c
> index 24bc6cc409..8679dc452d 100644
> --- a/ebpf/ebpf_rss.c
> +++ b/ebpf/ebpf_rss.c
> @@ -13,6 +13,8 @@
>   
>   #include "qemu/osdep.h"
>   #include "qemu/error-report.h"
> +#include "qapi/qapi-types-misc.h"
> +#include "qapi/qapi-commands-ebpf.h"
>   
>   #include <bpf/libbpf.h>
>   #include <bpf/bpf.h>
> @@ -21,6 +23,8 @@
>   
>   #include "ebpf/ebpf_rss.h"
>   #include "ebpf/rss.bpf.skeleton.h"
> +#include "ebpf/ebpf.h"
> +
>   #include "trace.h"
>   
>   void ebpf_rss_init(struct EBPFRSSContext *ctx)
> @@ -261,3 +265,5 @@ void ebpf_rss_unload(struct EBPFRSSContext *ctx)
>       ctx->map_toeplitz_key = -1;
>       ctx->map_indirections_table = -1;
>   }
> +
> +ebpf_binary_init(EBPF_PROGRAMID_RSS, rss_bpf__elf_bytes)
> diff --git a/ebpf/meson.build b/ebpf/meson.build
> index 2f627d6c7d..c5bf9295a2 100644
> --- a/ebpf/meson.build
> +++ b/ebpf/meson.build
> @@ -1 +1 @@
> -system_ss.add(when: libbpf, if_true: files('ebpf_rss.c'), if_false: files('ebpf_rss-stub.c'))
> +common_ss.add(when: libbpf, if_true: files('ebpf.c', 'ebpf_rss.c'), if_false: files('ebpf_rss-stub.c'))
> diff --git a/qapi/ebpf.json b/qapi/ebpf.json
> new file mode 100644
> index 0000000000..ba78407148
> --- /dev/null
> +++ b/qapi/ebpf.json
> @@ -0,0 +1,66 @@
> +# -*- Mode: Python -*-
> +# vim: filetype=python
> +#
> +# This work is licensed under the terms of the GNU GPL, version 2 or later.
> +# See the COPYING file in the top-level directory.
> +
> +##
> +# = eBPF Objects
> +#
> +# eBPF object is an ELF binary that contains the eBPF
> +# program and eBPF map description(BTF). Overall, eBPF
> +# object should contain the program and enough metadata
> +# to create/load eBPF with libbpf. As the eBPF maps/program
> +# should correspond to QEMU, the eBPF can't be used from
> +# different QEMU build.
> +#
> +# Currently, there is a possible eBPF for receive-side scaling (RSS).
> +#
> +##
> +
> +##
> +# @EbpfObject:
> +#
> +# An eBPF ELF object.
> +#
> +# @object: the eBPF object encoded in base64
> +#
> +# Since: 8.2

These need to be updated; we missed 8.2.