Re: Custom types in target description XMLs

[Simon Marchi <simon.marchi@polymtl.ca>]

On 2021-07-13 3:08 p.m., Shahab Vahedi via Gdb wrote:
> Hello,
> 
> I have difficulties defining a custom register type in a target
> description XML format. For the sake of argument, consider the
> 32-bit register below with it's own contrived fields:
> 
> bit:   31     ...     11  10  9   8  7  ...  0
>       ,-----------------.---.---.---.---------.
>       | dont_care_field | g | u | b | version |
>       `-----------------^---^---^---^---------'
> 
> I've come up with two definitions. One uses the "flag" and the
> other uses the "struct" type. However, both have their issues
> in representing the correct values. The definitions are as
> follows:
> 
> -----------------------------8<-----------------------------
> <?xml version="1.0"?>
> 
> <!DOCTYPE feature SYSTEM "gdb-target.dtd">
> <feature name="org.gnu.gdb.arc.fpu">
>   <flags id="flag_type" size="4">
>     <field name="version" start="0"  end="7"  type="int" />
>     <field name="b"       start="8"  end="8"  type="bool"/>
>     <field name="u"       start="9"  end="9"  type="bool"/>
>     <field name="g"       start="10" end="10" type="bool"/>
>     <field name=""        start="11" end="31"            />
>   </flags>
>   <struct id="struct_type" size="4">
>     <field name="version" start="0"  end="7"  type="int" />
>     <field name="b"       start="8"  end="8"  type="bool"/>
>     <field name="u"       start="9"  end="9"  type="bool"/>
>     <field name="g"       start="10" end="10" type="bool"/>
>     <field name=""        start="11" end="31"            />
>   </struct>
>   <reg name="f_reg" bitsize="32" type="flag_type"/>
>   <reg name="s_reg" bitsize="32" type="struct_type"/>
> </feature>
> ----------------------------->8-----------------------------
> 
> The value reported by GDBstub (QEMU) is 0x1337 for both registers:
> 
> 0  x   1    3    3    7
>      0001 0011 0011 0111
> 
> version: 0x37 (55)
> b: true
> u: true
> g: false
> 
> However, this is how GDB prints them:
> 
> ---------------------- [ gdb ] ----------------------
> (gdb) info reg $f_reg
> f_reg          0x1337              [ version=0 b u ]
> -----------------------------------------------------
> 
> Here, while "$f_reg" holds the correct value (0x1337), the
> "version" field is 0 instead of 55. "b", "u", and "g" fields
> are inferred correctly though.
> 
> ---------------------- [ gdb ] ----------------------
> (gdb) info reg $s_reg
> s_reg          {
>   version = 0x37,
>   b = 0xff,
>   u = 0xff,
>   g = 0x0
> } {
>   version = 55,
>   b = -1,
>   u = -1,
>   g = false
> }
> -----------------------------------------------------
> 
> The register with struct type on the other hand, interprets
> "version" correctly while misjudging the "boolean" fields that
> are set.
> 
> Am I doing something wrong or this could be a bug in parsing?
> If latter is the case, I will dive into the code to find the
> root cause. I just need someone to confirm it first.

Hi Shahab,

Given the lack of response, I would say that nobody knows that off-hand,
and you would need to dive into the code to check what is happening, if
it's a GDB bug or something else.

Simon