Re: [RFC 0/5] hmp, qmp: Add some commands to introspect virtio devices

[Marc-André Lureau]

Hi

On Thu, Apr 2, 2020 at 12:03 PM Laurent Vivier <address@hidden> wrote:
>
> This series introduces new QMP/HMP commands to dump the status
> of a a virtio device at different levels.

Looks nice, but my feeling is that we are growing qemu with debugging
facilities in general with HMP/QMP.

If this is only for debugging purposes, why don't we compile it only
when --enable-debug?

A guest that would need to be debugged could have its state loaded in
a debug version of qemu.

Alternatively, I think most of the data you provide here could
probably be introspected via gdb scripts.

Just some thoughts

>
> 1. Main command
>
>  HMP Only:
>
>      virtio [subcommand]
>
>    Example:
>
>     List all sub-commands:
>
>     (qemu) virtio
>     virtio query  -- List all available virtio devices
>     virtio status path -- Display status of a given virtio device
>     virtio queue-status path queue -- Display status of a given virtio queue
>     virtio queue-element path queue [index] -- Display element of a given 
> virtio queue
>
> 2. List available virtio devices in the machine
>
>   HMP Form:
>
>     virtio query
>
>   Example:
>
>     (qemu) virtio query
>     /machine/peripheral-anon/device[3]/virtio-backend [virtio-net]
>     /machine/peripheral-anon/device[1]/virtio-backend [virtio-serial]
>     /machine/peripheral-anon/device[0]/virtio-backend [virtio-blk]
>
>   QMP Form:
>
>     { 'command': 'query-virtio', 'returns': ['VirtioInfo'] }
>
>   Example:
>
>   -> { "execute": "query-virtio" }
>   <- { "return": [
>          {
>              "path": "/machine/peripheral-anon/device[3]/virtio-backend",
>              "type": "virtio-net"
>          },
>          {
>              "path": "/machine/peripheral-anon/device[1]/virtio-backend",
>              "type": "virtio-serial"
>          },
>          {
>              "path": "/machine/peripheral-anon/device[0]/virtio-backend",
>              "type": "virtio-blk"
>          }
>        ]
>      }
>
> 3. Display status of a given virtio device
>
>   HMP Form:
>
>     virtio status <path>
>
>   Example:
>
>     (qemu) virtio status /machine/peripheral-anon/device[3]/virtio-backend
>     /machine/peripheral-anon/device[3]/virtio-backend:
>       Device Id:        1
>       Guest features:   0x0000000130afffa7
>       Host features:    0x0000000179bfffe7
>       Backend features: 0x0000000000000000
>       Endianness:       little
>       VirtQueues:       3
>
>   QMP Form:
>
>     { 'command': 'virtio-status',
>       'data': { 'path': 'str' },
>       'returns': 'VirtioStatus'
>     }
>
>   Example:
>
>   -> { "execute": "virtio-status",
>        "arguments": {
>            "path": "/machine/peripheral-anon/device[3]/virtio-backend"
>        }
>     }
>   <- { "return": {
>            "backend_features": 0,
>            "guest_features": 5111807911,
>            "num_vqs": 3,
>            "host_features": 6337593319,
>            "device_endian": "little",
>            "device_id": 1
>        }
>      }
>
> 4. Display status of a given virtio queue
>
>   HMP Form:
>
>     virtio queue-status <path> <queue>
>
>   Example:
>
>     (qemu) virtio queue-status 
> /machine/peripheral-anon/device[3]/virtio-backend 0
>     /machine/peripheral-anon/device[3]/virtio-backend:
>       index:                0
>       inuse:                0
>       last_avail_idx:       61
>       shadow_avail_idx:     292
>       signalled_used:       61
>       signalled_used_valid: 1
>       VRing:
>         num:         256
>         num_default: 256
>         align:       4096
>         desc:        0x000000006c352000
>         avail:       0x000000006c353000
>         used:        0x000000006c353240
>
>   QMP Form:
>
>     { 'command': 'virtio-queue-status',
>       'data': { 'path': 'str', 'queue': 'uint16' },
>       'returns': 'VirtQueueStatus'
>     }
>
>   Example:
>
>   -> { "execute": "virtio-queue-status",
>        "arguments": {
>            "path": "/machine/peripheral-anon/device[3]/virtio-backend",
>            "queue": 0
>        }
>     }
>   <- { "return": {
>        "signalled_used": 373,
>        "inuse": 0,
>        "vring_desc": 864411648,
>        "vring_num_default": 256,
>        "signalled_used_valid": 1,
>        "vring_avail": 864415744,
>        "last_avail_idx": 373,
>        "queue_index": 0,
>        "vring_used": 864416320,
>        "shadow_avail_idx": 619,
>        "used_idx": 373,
>        "vring_num": 256,
>        "vring_align": 4096
>        }
>      }
>
> 5. Display element of a given virtio queue
>
>   HMP Form:
>
>     virtio queue-element <path> <queue> [index]
>
>   Example:
>
>     Dump the information of the head element of the first queue of
>     the first virtio device::
>
>       (qemu) virtio 
> queue-element/machine/peripheral-anon/device[3]/virtio-backend 0
>       index:  67
>       ndescs: 1
>       descs:  addr 0x6fe69800 len 1536 (write-only)
>
>       (qemu) xp/128bx 0x6fe69800
>       000000006fe69800: 0x02 0x00 0x00 0x00 0x00 0x00 0x00 0x00
>       000000006fe69808: 0x00 0x00 0x01 0x00 0x52 0x54 0x00 0x12
>       000000006fe69810: 0x34 0x56 0x52 0x54 0x00 0x09 0x51 0xde
>       000000006fe69818: 0x08 0x00 0x45 0x00 0x00 0x4c 0x8f 0x32
>
>     device[3] is a virtio-net device and we can see in the element buffer the
>     MAC address of the card::
>
>       [root@localhost ~]# ip link show ens4
>       2: ens4: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel 
> state UP m0
>           link/ether 52:54:00:12:34:56 brd ff:ff:ff:ff:ff:ff
>
>     and the MAC address of the gateway::
>
>       [root@localhost ~]# arp -a
>       _gateway (192.168.122.1) at 52:54:00:09:51:de [ether] on ens4
>
>   QMP Form:
>
>     { 'command': 'virtio-queue-element',
>       'data': { 'path': 'str', 'queue': 'uint16', '*index': 'uint16' },
>       'returns': 'VirtioQueueElement'
>     }
>
>   Example:
>
>   -> { "execute": "virtio-queue-element",
>        "arguments": {
>            "path": "/machine/peripheral-anon/device[3]/virtio-backend",
>            "queue": 0
>        }
>     }
>   -> { "return": {
>           "index": 109,
>           "len": 0,
>           "ndescs": 1,
>           "descs": [
>               { "flags": 2, "len": 2048, "addr": 853145600 }
>           ]
>        }
>     }
>
> Laurent Vivier (5):
>   qmp: add QMP command query-virtio
>   qmp: add QMP command virtio-status
>   qmp: add QMP command virtio-queue-status
>   qmp: add QMP command virtio-queue-element
>   hmp: add virtio commands
>
>  Makefile                   |   2 +-
>  Makefile.target            |   7 +-
>  docs/system/monitor.rst    |   2 +
>  hmp-commands-virtio.hx     | 148 ++++++++++++++++++
>  hmp-commands.hx            |  10 ++
>  hw/virtio/Makefile.objs    |   2 +
>  hw/virtio/virtio-stub.c    |  32 ++++
>  hw/virtio/virtio.c         | 313 +++++++++++++++++++++++++++++++++++++
>  include/hw/virtio/virtio.h |   1 +
>  include/monitor/hmp.h      |   4 +
>  monitor/misc.c             |  17 ++
>  qapi/Makefile.objs         |   2 +-
>  qapi/qapi-schema.json      |   1 +
>  qapi/virtio.json           | 302 +++++++++++++++++++++++++++++++++++
>  tests/qtest/qmp-cmd-test.c |   1 +
>  15 files changed, 840 insertions(+), 4 deletions(-)
>  create mode 100644 hmp-commands-virtio.hx
>  create mode 100644 hw/virtio/virtio-stub.c
>  create mode 100644 qapi/virtio.json
>
> --
> 2.25.1
>
>


-- 
Marc-André Lureau