Re: Questions on XML register descriptions

["H. Peter Anvin via Gdb" <gdb@sourceware.org>]

On 12/29/23 05:41, Luis Machado via Gdb wrote:
> 
> Ok, I went to check this again, as I had forgotten what gdb did. It looks like you're right. From the code comment about handling the g packet reply from the remote:
> 
>    /* If this is smaller than we guessed the 'g' packet would be,
>       update our records.  A 'g' reply that doesn't include a register's
>       value implies either that the register is not available, or that
>       the 'p' packet must be used.  */
> 
> So it looks like you can force gdb to fallback to the p packet by making the g reply packet of minimum size.
> 

That's great.

>>
>> The compiler communicating it isn't essential here as I'm not trying to expose things that are visible to the compiler. What I want to do is to be able to use gdb to examine/access non-CPU-visible address spaces, like the physical RAM even when it is paged out, or the I/O address space.
>>
>> The problem becomes that the type of the pointer is lost if it is simply converted to an integer, and casting it back to a pointer causes it to be truncated back to 16 bits.
> 
> Yeah, so this is the type of knowledge gdb must have in order to handle things correctly. Technically you could handle things correctly from the remote's side by inspecing the memory read/write packets, determining what the address space should be, what the pointer size might be and then going to fetch that information from wherever address space it belongs to.
> 
>>
>> I tried doing a Python script like this, but it ran into the above problem (plus the connection problem mentioned below):
>>
> 
> What particular error do you get, out of curiosity?
> 
>> #!/usr/bin/python
>>
>> import gdb
>>
>> class Addrspace (gdb.Function):
>>      """Convert an address or a pointer to the named address space."""
>>      def __init__ (self, name, base, mask):
>>          self.name = name
>>          self.mask = mask
>>          self.base = base
>>          arch = gdb.selected_inferior().architecture()
>>          bits = 8
>>          while (mask|base) >> bits:
>>              bits <<= 1
>>          self.itype = arch.integer_type(bits, False)
>>          super (Addrspace, self).__init__ (name)
>>
>>      def invoke (self, val):
>>          type = val.type
>>          return ((val.cast(self.itype) & self.mask) \
>>           + self.base).cast(type)
>>
>> _io = Addrspace ("io", 0x01000000, 0xffff)
>>

The problem is that when the address is cast back to a pointer, it is 
truncated to the pointer size.

Having thought about this some more I wonder if it is possible to create 
a new Type similar to a normal pointer but larger, and use that as the 
new type instead of val.type.

	-hpa