Re: [PATCH for-7.2 v4 15/21] qmp/hmp, device_tree.c: introduce 'info fdt' command

[David Gibson]

On Mon, Aug 29, 2022 at 07:00:55PM -0300, Daniel Henrique Barboza wrote:
> 
> 
> On 8/29/22 00:34, David Gibson wrote:
> > On Fri, Aug 26, 2022 at 11:11:44AM -0300, Daniel Henrique Barboza wrote:
> > > Reading the FDT requires that the user saves the fdt_blob and then use
> > > 'dtc' to read the contents. Saving the file and using 'dtc' is a strong
> > > use case when we need to compare two FDTs, but it's a lot of steps if
> > > you want to do quick check on a certain node or property.
> > > 
> > > 'info fdt' retrieves FDT nodes (and properties, later on) and print it
> > > to the user. This can be used to check the FDT on running machines
> > > without having to save the blob and use 'dtc'.
> > > 
> > > The implementation is based on the premise that the machine thas a FDT
> > > created using libfdt and pointed by 'machine->fdt'. As long as this
> > > pre-requisite is met the machine should be able to support it.
> > > 
> > > For now we're going to add the required QMP/HMP boilerplate and the
> > > capability of printing the name of the properties of a given node. Next
> > > patches will extend 'info fdt' to be able to print nodes recursively,
> > > and then individual properties.
> > > 
> > > This command will always be executed in-band (i.e. holding BQL),
> > > avoiding potential race conditions with machines that might change the
> > > FDT during runtime (e.g. PowerPC 'pseries' machine).
> > > 
> > > 'info fdt' is not something that we expect to be used aside from 
> > > debugging,
> > > so we're implementing it in QMP as 'x-query-fdt'.
> > > 
> > > This is an example of 'info fdt' fetching the '/chosen' node of the
> > > pSeries machine:
> > > 
> > > (qemu) info fdt /chosen
> > > chosen {
> > >      ibm,architecture-vec-5;
> > >      rng-seed;
> > >      ibm,arch-vec-5-platform-support;
> > >      linux,pci-probe-only;
> > >      stdout-path;
> > >      linux,stdout-path;
> > >      qemu,graphic-depth;
> > >      qemu,graphic-height;
> > >      qemu,graphic-width;
> > > };
> > > 
> > > And the same node for the aarch64 'virt' machine:
> > > 
> > > (qemu) info fdt /chosen
> > > chosen {
> > >      stdout-path;
> > >      rng-seed;
> > >      kaslr-seed;
> > > };
> > 
> > So, I'm reasonably convinced allowing dumping the whole dtb from
> > qmp/hmp is useful.  I'm less convined that info fdt is worth the
> > additional complexity it incurs.  Note that as well as being able to
> > decompile a whole dtb using dtc, you can also extract and list
> > specific properties from a dtb blob using the 'fdtget' tool which is
> > part of the dtc tree.
> 
> What's your opinion on patch 21/21, where 'dumpdtb' can write a formatted
> FDT in a file with an extra option? That was possible because of the
> format helpers introduced for 'info fdt'. The idea is that since we're
> able to format a FDT in DTS format, we can also write the FDT in text
> format without relying on DTC to decode it.

Since it's mostly the same code, I think it's reasonable to throw in
if the info fdt stuff is there, but I don't think it's worth including
without that.  As a whole, I remain dubious that (info fdt + dumpdts)
is worth the complexity cost.

People with more practical experience debugging the embedded ARM
platforms might have a different opinion if they thing info fdt would
be really useful though.

> If we think that this 'dumpdtb' capability is worth having, I can respin
> the patches without 'info fdt' but adding these helpers to enable this
> 'dumpdtb' support. If not, then we can just remove patches 15-21 and
> be done with it.
> 
> 
> Thanks,
> 
> 
> Daniel
> 
> > 
> > > 
> > > Cc: Dr. David Alan Gilbert <dgilbert@redhat.com>
> > > Acked-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> > > Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
> > > ---
> > >   hmp-commands-info.hx         | 13 ++++++++++
> > >   include/monitor/hmp.h        |  1 +
> > >   include/sysemu/device_tree.h |  4 +++
> > >   monitor/hmp-cmds.c           | 13 ++++++++++
> > >   monitor/qmp-cmds.c           | 12 +++++++++
> > >   qapi/machine.json            | 19 +++++++++++++++
> > >   softmmu/device_tree.c        | 47 ++++++++++++++++++++++++++++++++++++
> > >   7 files changed, 109 insertions(+)
> > > 
> > > diff --git a/hmp-commands-info.hx b/hmp-commands-info.hx
> > > index 188d9ece3b..743b48865d 100644
> > > --- a/hmp-commands-info.hx
> > > +++ b/hmp-commands-info.hx
> > > @@ -921,3 +921,16 @@ SRST
> > >     ``stats``
> > >       Show runtime-collected statistics
> > >   ERST
> > > +
> > > +    {
> > > +        .name       = "fdt",
> > > +        .args_type  = "nodepath:s",
> > > +        .params     = "nodepath",
> > > +        .help       = "show firmware device tree node given its path",
> > > +        .cmd        = hmp_info_fdt,
> > > +    },
> > > +
> > > +SRST
> > > +  ``info fdt``
> > > +    Show a firmware device tree node given its path. Requires libfdt.
> > > +ERST
> > > diff --git a/include/monitor/hmp.h b/include/monitor/hmp.h
> > > index d7f324da59..c0883dd1e3 100644
> > > --- a/include/monitor/hmp.h
> > > +++ b/include/monitor/hmp.h
> > > @@ -135,6 +135,7 @@ 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_info_fdt(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 bf7684e4ed..057d13e397 100644
> > > --- a/include/sysemu/device_tree.h
> > > +++ b/include/sysemu/device_tree.h
> > > @@ -14,6 +14,8 @@
> > >   #ifndef DEVICE_TREE_H
> > >   #define DEVICE_TREE_H
> > > +#include "qapi/qapi-types-common.h"
> > > +
> > >   void *create_device_tree(int *sizep);
> > >   void *load_device_tree(const char *filename_path, int *sizep);
> > >   #ifdef CONFIG_LINUX
> > > @@ -137,6 +139,8 @@ int qemu_fdt_add_path(void *fdt, const char *path);
> > >   void qemu_fdt_dumpdtb(void *fdt, int size);
> > >   void qemu_fdt_qmp_dumpdtb(const char *filename, Error **errp);
> > > +HumanReadableText *qemu_fdt_qmp_query_fdt(const char *nodepath,
> > > +                                          Error **errp);
> > >   /**
> > >    * qemu_fdt_setprop_sized_cells_from_array:
> > > diff --git a/monitor/hmp-cmds.c b/monitor/hmp-cmds.c
> > > index 1c7bfd3b9d..93a4103afa 100644
> > > --- a/monitor/hmp-cmds.c
> > > +++ b/monitor/hmp-cmds.c
> > > @@ -2484,3 +2484,16 @@ void hmp_dumpdtb(Monitor *mon, const QDict *qdict)
> > >           hmp_handle_error(mon, local_err);
> > >       }
> > >   }
> > > +
> > > +void hmp_info_fdt(Monitor *mon, const QDict *qdict)
> > > +{
> > > +    const char *nodepath = qdict_get_str(qdict, "nodepath");
> > > +    Error *err = NULL;
> > > +    g_autoptr(HumanReadableText) info = qmp_x_query_fdt(nodepath, &err);
> > > +
> > > +    if (hmp_handle_error(mon, err)) {
> > > +        return;
> > > +    }
> > > +
> > > +    monitor_printf(mon, "%s", info->human_readable_text);
> > > +}
> > > diff --git a/monitor/qmp-cmds.c b/monitor/qmp-cmds.c
> > > index 8415aca08c..db2c6aa7da 100644
> > > --- a/monitor/qmp-cmds.c
> > > +++ b/monitor/qmp-cmds.c
> > > @@ -603,9 +603,21 @@ void qmp_dumpdtb(const char *filename, Error **errp)
> > >   {
> > >       return qemu_fdt_qmp_dumpdtb(filename, errp);
> > >   }
> > > +
> > > +HumanReadableText *qmp_x_query_fdt(const char *nodepath, Error **errp)
> > > +{
> > > +    return qemu_fdt_qmp_query_fdt(nodepath, errp);
> > > +}
> > >   #else
> > >   void qmp_dumpdtb(const char *filename, Error **errp)
> > >   {
> > >       error_setg(errp, "dumpdtb requires libfdt");
> > >   }
> > > +
> > > +HumanReadableText *qmp_x_query_fdt(const char *nodepath, Error **errp)
> > > +{
> > > +    error_setg(errp, "this command requires libfdt");
> > > +
> > > +    return NULL;
> > > +}
> > >   #endif
> > > diff --git a/qapi/machine.json b/qapi/machine.json
> > > index aeb013f3dd..96cff541ca 100644
> > > --- a/qapi/machine.json
> > > +++ b/qapi/machine.json
> > > @@ -1681,3 +1681,22 @@
> > >   ##
> > >   { 'command': 'dumpdtb',
> > >     'data': { 'filename': 'str' } }
> > > +
> > > +##
> > > +# @x-query-fdt:
> > > +#
> > > +# Query for FDT element (node or property). Requires 'libfdt'.
> > > +#
> > > +# @nodepath: the path of the FDT node to be retrieved
> > > +#
> > > +# Features:
> > > +# @unstable: This command is meant for debugging.
> > > +#
> > > +# Returns: FDT node
> > > +#
> > > +# Since: 7.2
> > > +##
> > > +{ 'command': 'x-query-fdt',
> > > +  'data': { 'nodepath': 'str' },
> > > +  'returns': 'HumanReadableText',
> > > +  'features': [ 'unstable' ]  }
> > > diff --git a/softmmu/device_tree.c b/softmmu/device_tree.c
> > > index cd487ddd4d..6b15f6ace2 100644
> > > --- a/softmmu/device_tree.c
> > > +++ b/softmmu/device_tree.c
> > > @@ -18,6 +18,7 @@
> > >   #endif
> > >   #include "qapi/error.h"
> > > +#include "qapi/type-helpers.h"
> > >   #include "qemu/error-report.h"
> > >   #include "qemu/option.h"
> > >   #include "qemu/bswap.h"
> > > @@ -661,3 +662,49 @@ void qemu_fdt_qmp_dumpdtb(const char *filename, 
> > > Error **errp)
> > >       error_setg(errp, "Error when saving machine FDT to file %s", 
> > > filename);
> > >   }
> > > +
> > > +static void fdt_format_node(GString *buf, int node, int depth)
> > > +{
> > > +    const struct fdt_property *prop = NULL;
> > > +    const char *propname = NULL;
> > > +    void *fdt = current_machine->fdt;
> > > +    int padding = depth * 4;
> > > +    int property = 0;
> > > +    int prop_size;
> > > +
> > > +    g_string_append_printf(buf, "%*s%s {\n", padding, "",
> > > +                           fdt_get_name(fdt, node, NULL));
> > > +
> > > +    padding += 4;
> > > +
> > > +    fdt_for_each_property_offset(property, fdt, node) {
> > > +        prop = fdt_get_property_by_offset(fdt, property, &prop_size);
> > > +        propname = fdt_string(fdt, fdt32_to_cpu(prop->nameoff));
> > > +
> > > +        g_string_append_printf(buf, "%*s%s;\n", padding, "", propname);
> > > +    }
> > > +
> > > +    padding -= 4;
> > > +    g_string_append_printf(buf, "%*s};\n", padding, "");
> > > +}
> > > +
> > > +HumanReadableText *qemu_fdt_qmp_query_fdt(const char *nodepath, Error 
> > > **errp)
> > > +{
> > > +    g_autoptr(GString) buf = g_string_new("");
> > > +    int node;
> > > +
> > > +    if (!current_machine->fdt) {
> > > +        error_setg(errp, "Unable to find the machine FDT");
> > > +        return NULL;
> > > +    }
> > > +
> > > +    node = fdt_path_offset(current_machine->fdt, nodepath);
> > > +    if (node < 0) {
> > > +        error_setg(errp, "node '%s' not found in FDT", nodepath);
> > > +        return NULL;
> > > +    }
> > > +
> > > +    fdt_format_node(buf, node, 0);
> > > +
> > > +    return human_readable_text_from_str(buf);
> > > +}
> > 
> 

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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