Re: [PATCH for-7.2 v2 14/20] qmp/hmp, device_tree.c: introduce dumpdtb

[Daniel Henrique Barboza]

On 8/15/22 15:31, Dr. David Alan Gilbert wrote:

* Daniel Henrique Barboza (danielhb...@gmail.com) wrote:

On 8/8/22 00:30, David Gibson wrote:

On Fri, Aug 05, 2022 at 06:39:42AM -0300, Daniel Henrique Barboza wrote:

To save the FDT blob we have the '-machine dumpdtb=<file>' property.
With this property set, the machine saves the FDT in <file> and exit.
The created file can then be converted to plain text dts format using
'dtc'.

There's nothing particularly sophisticated into saving the FDT that
can't be done with the machine at any state, as long as the machine has
a valid FDT to be saved.

The 'dumpdtb' command receives a 'filename' paramenter and, if a valid
FDT is available, it'll save it in a file 'filename'. In short, this is
a '-machine dumpdtb' that can be fired on demand via QMP/HMP.

A valid FDT consists of a FDT that was created using libfdt being
retrieved via 'current_machine->fdt' in device_tree.c. This condition is
met by most FDT users in QEMU.

On spapr at least, the fdt can change at runtime (due to hotplugs).
So we need to think about concurrency between fdt updates and dumping
it with this command.  I'm assuming it's protected by the BQL, but I'm
wondering if we should outright state that somewhere for clarity.

Your assumption is right. It is protected by BQL since it's always executed
in-band. To not hold BQL we would need to declare an extra flag "allow-oob"
to execute the command out-of-band.

I'll mention in the commit msg that we're holding BQL. I'll do the same for
the 'info fdt' commit msg.

Is the update of the FDT also protected that way?

Yes. At least for the pseries machine, the only machine we have ATM that updates
the FDT after boot, without hotplug/unplug, and uses libfdt.

The update of the pseries FDT occurs in the h_client_architecture_support hcall,
which is always called holding BQL (either via kvm_arch_handle_exit() or
emulate_spapr_hypercall).

(I guess hotplug commands are also protected by the bql - but is the
hotplug completion?)

Hotplug completion is protected by BQL in all machines, as far as I know.


Thanks,


Daniel

Dave

Thanks,


Daniel

Cc: Dr. David Alan Gilbert <dgilb...@redhat.com>
Signed-off-by: Daniel Henrique Barboza <danielhb...@gmail.com>
---
   hmp-commands.hx              | 13 +++++++++++++
   include/monitor/hmp.h        |  1 +
   include/sysemu/device_tree.h |  1 +
   monitor/hmp-cmds.c           | 12 ++++++++++++
   monitor/qmp-cmds.c           | 13 +++++++++++++
   qapi/machine.json            | 17 +++++++++++++++++
   softmmu/device_tree.c        | 18 ++++++++++++++++++
   7 files changed, 75 insertions(+)

diff --git a/hmp-commands.hx b/hmp-commands.hx
index 182e639d14..d2554e9701 100644
--- a/hmp-commands.hx
+++ b/hmp-commands.hx
@@ -1800,3 +1800,16 @@ ERST
                         "\n\t\t\t\t\t limit on a specified virtual cpu",
           .cmd        = hmp_cancel_vcpu_dirty_limit,
       },
+
+SRST
+``dumpdtb`` *filename*
+  Save the FDT in the 'filename' file to be decoded using dtc.
+  Requires 'libfdt' support.
+ERST
+    {
+        .name       = "dumpdtb",
+        .args_type  = "filename:s",
+        .params     = "[filename] file to save the FDT",
+        .help       = "save the FDT in the 'filename' file to be decoded using 
dtc",
+        .cmd        = hmp_dumpdtb,
+    },
diff --git a/include/monitor/hmp.h b/include/monitor/hmp.h
index a618eb1e4e..d7f324da59 100644
--- a/include/monitor/hmp.h
+++ b/include/monitor/hmp.h
@@ -134,6 +134,7 @@ void hmp_calc_dirty_rate(Monitor *mon, const QDict *qdict);
   void hmp_set_vcpu_dirty_limit(Monitor *mon, const QDict *qdict);
   void hmp_cancel_vcpu_dirty_limit(Monitor *mon, const QDict *qdict);
   void hmp_info_vcpu_dirty_limit(Monitor *mon, const QDict *qdict);
+void hmp_dumpdtb(Monitor *mon, const QDict *qdict);
   void hmp_human_readable_text_helper(Monitor *mon,
                                       HumanReadableText *(*qmp_handler)(Error 
**));
   void hmp_info_stats(Monitor *mon, const QDict *qdict);
diff --git a/include/sysemu/device_tree.h b/include/sysemu/device_tree.h
index ef060a9759..bf7684e4ed 100644
--- a/include/sysemu/device_tree.h
+++ b/include/sysemu/device_tree.h
@@ -136,6 +136,7 @@ int qemu_fdt_add_path(void *fdt, const char *path);
       } while (0)
   void qemu_fdt_dumpdtb(void *fdt, int size);
+void qemu_fdt_qmp_dumpdtb(const char *filename, Error **errp);
   /**
    * qemu_fdt_setprop_sized_cells_from_array:
diff --git a/monitor/hmp-cmds.c b/monitor/hmp-cmds.c
index c6cd6f91dd..d23ec85f9d 100644
--- a/monitor/hmp-cmds.c
+++ b/monitor/hmp-cmds.c
@@ -2472,3 +2472,15 @@ exit:
   exit_no_print:
       error_free(err);
   }
+
+void hmp_dumpdtb(Monitor *mon, const QDict *qdict)
+{
+    const char *filename = qdict_get_str(qdict, "filename");
+    Error *local_err = NULL;
+
+    qmp_dumpdtb(filename, &local_err);
+
+    if (local_err) {
+        error_report_err(local_err);
+    }
+}
diff --git a/monitor/qmp-cmds.c b/monitor/qmp-cmds.c
index 7314cd813d..8415aca08c 100644
--- a/monitor/qmp-cmds.c
+++ b/monitor/qmp-cmds.c
@@ -45,6 +45,7 @@
   #include "hw/intc/intc.h"
   #include "hw/rdma/rdma.h"
   #include "monitor/stats.h"
+#include "sysemu/device_tree.h"
   NameInfo *qmp_query_name(Error **errp)
   {
@@ -596,3 +597,15 @@ bool apply_str_list_filter(const char *string, strList 
*list)
       }
       return false;
   }
+
+#ifdef CONFIG_FDT
+void qmp_dumpdtb(const char *filename, Error **errp)
+{
+    return qemu_fdt_qmp_dumpdtb(filename, errp);
+}
+#else
+void qmp_dumpdtb(const char *filename, Error **errp)
+{
+    error_setg(errp, "dumpdtb requires libfdt");
+}
+#endif
diff --git a/qapi/machine.json b/qapi/machine.json
index 6afd1936b0..aeb013f3dd 100644
--- a/qapi/machine.json
+++ b/qapi/machine.json
@@ -1664,3 +1664,20 @@
        '*size': 'size',
        '*max-size': 'size',
        '*slots': 'uint64' } }
+
+##
+# @dumpdtb:
+#
+# Save the FDT in dtb format. Requires 'libfdt' support.
+#
+# @filename: name of the FDT file to be created
+#
+# Since: 7.2
+#
+# Example:
+#   {"execute": "dumpdtb"}
+#    "arguments": { "filename": "/tmp/fdt.dtb" } }
+#
+##
+{ 'command': 'dumpdtb',
+  'data': { 'filename': 'str' } }
diff --git a/softmmu/device_tree.c b/softmmu/device_tree.c
index 6ca3fad285..cd487ddd4d 100644
--- a/softmmu/device_tree.c
+++ b/softmmu/device_tree.c
@@ -643,3 +643,21 @@ out:
       g_free(propcells);
       return ret;
   }
+
+void qemu_fdt_qmp_dumpdtb(const char *filename, Error **errp)
+{
+    int size;
+
+    if (!current_machine->fdt) {
+        error_setg(errp, "Unable to find the machine FDT");
+        return;
+    }
+
+    size = fdt_totalsize(current_machine->fdt);
+
+    if (g_file_set_contents(filename, current_machine->fdt, size, NULL)) {
+        return;
+    }
+
+    error_setg(errp, "Error when saving machine FDT to file %s", filename);
+}