[help] Issues with 'g' packet and MIPS - gdb interprets the packet reply wrong

[help] Issues with 'g' packet and MIPS - gdb interprets the packet reply wrong

[Christopher Bainbridge]

Hi all,

I am trying to implement a remote stub for a MIPS cpu (using GDB version 
7.8). When GDB asks for the general registers using  the 'g' packet, I 
reply with:

00000000000000000d01000000000000fffdffff00000000000000000080c0bf30000000f0fec0bf000000002e0000000000000000000000000000008080808000000000000000000000000000000000000000000000000000000000000000000000000000000000000001800000000000000000e8fec0bf00000000702ec0bf0000000000000000000000000000000000000000702ec0bf

As each register is 32 bits (represented by 8 hex characters), this 
should be all the registers up to and including the PC.

However, GDB prints this out:

info reg
           zero       at       v0       v1       a0       a1 a2       a3
  R0   00000000 0000010d fffffdff 00000000 00000030 00000000 00000000 
00000000
             t0       t1       t2       t3       t4       t5 t6       t7
  R8   00000000 00000000 00000000 00000000 00000000 80010000 00000000 
00000000
             s0       s1       s2       s3       s4       s5 s6       s7
Sending packet: $p13#d4...Ack
Packet received: 00000000
Sending packet: $p14#d5...Ack
Packet received: 00000000
Sending packet: $p15#d6...Ack
Packet received: 00000000
Sending packet: $p16#d7...Ack
Packet received: 00000000
Sending packet: $p17#d8...Ack
Packet received: 00000000
  R16  00000000 00000000 00000000 00000000 00000000 00000000 00000000 
00000000
             t8       t9       k0       k1       gp       sp s8       ra
Sending packet: $p18#d9...Ack
Packet received: 00000000
Sending packet: $p19#da...Ack
Packet received: 00000000
Sending packet: $p1a#02...Ack
Packet received: 00000180
Sending packet: $p1b#03...Ack

etc

This looks to be that it is determining the size of each register 
incorrectly, and is thus asking for more registers using the 'p' packet.

Is this a bug on my end or in GDB?
I use the command

set processor mips:14000

beforehand, as this is the processor we're using.

Thanks,
Chris

Re: [help] Issues with 'g' packet and MIPS - gdb interprets the packet reply wrong

[Maciej W. Rozycki]

Hi Christopher,

> I am trying to implement a remote stub for a MIPS cpu (using GDB version 7.8).
> When GDB asks for the general registers using  the 'g' packet, I reply with:
> 
> 00000000000000000d01000000000000fffdffff00000000000000000080c0bf30000000f0fec0bf000000002e0000000000000000000000000000008080808000000000000000000000000000000000000000000000000000000000000000000000000000000000000001800000000000000000e8fec0bf00000000702ec0bf0000000000000000000000000000000000000000702ec0bf
> 
> As each register is 32 bits (represented by 8 hex characters), this should be
> all the registers up to and including the PC.

 What makes you assume the registers are 32 bits each?

> However, GDB prints this out:
> 
> info reg
>           zero       at       v0       v1       a0       a1 a2       a3
>  R0   00000000 0000010d fffffdff 00000000 00000030 00000000 00000000 00000000
>             t0       t1       t2       t3       t4       t5 t6       t7
>  R8   00000000 00000000 00000000 00000000 00000000 80010000 00000000 00000000
>             s0       s1       s2       s3       s4       s5 s6       s7
> Sending packet: $p13#d4...Ack
> Packet received: 00000000
> Sending packet: $p14#d5...Ack
> Packet received: 00000000
> Sending packet: $p15#d6...Ack
> Packet received: 00000000
> Sending packet: $p16#d7...Ack
> Packet received: 00000000
> Sending packet: $p17#d8...Ack
> Packet received: 00000000
>  R16  00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
>             t8       t9       k0       k1       gp       sp s8       ra
> Sending packet: $p18#d9...Ack
> Packet received: 00000000
> Sending packet: $p19#da...Ack
> Packet received: 00000000
> Sending packet: $p1a#02...Ack
> Packet received: 00000180
> Sending packet: $p1b#03...Ack
> 
> etc
> 
> This looks to be that it is determining the size of each register incorrectly,
> and is thus asking for more registers using the 'p' packet.
> 
> Is this a bug on my end or in GDB?
> I use the command
> 
> set processor mips:14000
> 
> beforehand, as this is the processor we're using.

 You mean:

(gdb) set architecture mips:14000

I presume, right?

 The R14000 is a 64-bit processor so its registers are 64-bit and will be 
treated as such by default by GDB.  You may be able to limit the width of 
registers expected by selecting a 32-bit processor instead or by selecting 
a 32-bit ABI such as `o32'.  The latter can be done with:

(gdb) set mips abi o32

or by selecting a file to debug that has been built for that ABI.

 Please note that this is a grey area though, with a bare-metal stub you 
should be really exchanging registers with GDB in their native sizes and 
letting GDB truncate and extend them as required depending on the ABI used 
by the program being debugged.  GDB is already capable of doing that, 
however in order to make use of that capability both the stub and GDB 
would have to support XML register descriptions which is something that 
owing to the vast number of CP0 register set variants in the MIPS 
architecture has never been implemented.  So in fact you may be hitting 
problems regardless of the ABI selection noted above.

 You can always determine the widths of registers GDB expects with the:

(gdb) maintenance print registers

command -- see the `Type' column on the right for the internal type used 
and note that the registers exchanged with a remote stub are those in the 
low half of indices (`Nr' == `Rel'), the so called "raw registers".

 HTH,

  Maciej

Re: [help] Issues with 'g' packet and MIPS - gdb interprets the packet reply wrong

[Christopher Bainbridge]

Many thanks!

It turns out that I mistook the M14000 to be the same as the M14k... 
which, annoyingly, they aren't.

So doing:

     set architecture mips:isa32r2

has solved my problem.

Thanks a lot!
Chris

On 21/10/14 21:34, Maciej W. Rozycki wrote:
> Hi Christopher,
>
>> I am trying to implement a remote stub for a MIPS cpu (using GDB version 7.8).
>> When GDB asks for the general registers using  the 'g' packet, I reply with:
>>
>> 00000000000000000d01000000000000fffdffff00000000000000000080c0bf30000000f0fec0bf000000002e0000000000000000000000000000008080808000000000000000000000000000000000000000000000000000000000000000000000000000000000000001800000000000000000e8fec0bf00000000702ec0bf0000000000000000000000000000000000000000702ec0bf
>>
>> As each register is 32 bits (represented by 8 hex characters), this should be
>> all the registers up to and including the PC.
>   What makes you assume the registers are 32 bits each?
>
>> However, GDB prints this out:
>>
>> info reg
>>            zero       at       v0       v1       a0       a1 a2       a3
>>   R0   00000000 0000010d fffffdff 00000000 00000030 00000000 00000000 00000000
>>              t0       t1       t2       t3       t4       t5 t6       t7
>>   R8   00000000 00000000 00000000 00000000 00000000 80010000 00000000 00000000
>>              s0       s1       s2       s3       s4       s5 s6       s7
>> Sending packet: $p13#d4...Ack
>> Packet received: 00000000
>> Sending packet: $p14#d5...Ack
>> Packet received: 00000000
>> Sending packet: $p15#d6...Ack
>> Packet received: 00000000
>> Sending packet: $p16#d7...Ack
>> Packet received: 00000000
>> Sending packet: $p17#d8...Ack
>> Packet received: 00000000
>>   R16  00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
>>              t8       t9       k0       k1       gp       sp s8       ra
>> Sending packet: $p18#d9...Ack
>> Packet received: 00000000
>> Sending packet: $p19#da...Ack
>> Packet received: 00000000
>> Sending packet: $p1a#02...Ack
>> Packet received: 00000180
>> Sending packet: $p1b#03...Ack
>>
>> etc
>>
>> This looks to be that it is determining the size of each register incorrectly,
>> and is thus asking for more registers using the 'p' packet.
>>
>> Is this a bug on my end or in GDB?
>> I use the command
>>
>> set processor mips:14000
>>
>> beforehand, as this is the processor we're using.
>   You mean:
>
> (gdb) set architecture mips:14000
>
> I presume, right?
>
>   The R14000 is a 64-bit processor so its registers are 64-bit and will be
> treated as such by default by GDB.  You may be able to limit the width of
> registers expected by selecting a 32-bit processor instead or by selecting
> a 32-bit ABI such as `o32'.  The latter can be done with:
>
> (gdb) set mips abi o32
>
> or by selecting a file to debug that has been built for that ABI.
>
>   Please note that this is a grey area though, with a bare-metal stub you
> should be really exchanging registers with GDB in their native sizes and
> letting GDB truncate and extend them as required depending on the ABI used
> by the program being debugged.  GDB is already capable of doing that,
> however in order to make use of that capability both the stub and GDB
> would have to support XML register descriptions which is something that
> owing to the vast number of CP0 register set variants in the MIPS
> architecture has never been implemented.  So in fact you may be hitting
> problems regardless of the ABI selection noted above.
>
>   You can always determine the widths of registers GDB expects with the:
>
> (gdb) maintenance print registers
>
> command -- see the `Type' column on the right for the internal type used
> and note that the registers exchanged with a remote stub are those in the
> low half of indices (`Nr' == `Rel'), the so called "raw registers".
>
>   HTH,
>
>    Maciej