Re: [PATCH v9 21/29] record-btrace: provide xfer_partial target method

[Pedro Alves <palves@redhat.com>]

On 12/20/2013 01:54 PM, Metzger, Markus T wrote:
>> -----Original Message-----
>> From: Metzger, Markus T
>> Sent: Friday, December 20, 2013 2:37 PM
>> To: Pedro Alves
> 
> 
>> I can try to make raw_memorx_xfer_partial return on
>> TARGET_XFER_E_UNAVAILABLE.  This will be more intrusive, though.
> 
> I did not get any regressions on 64bit IA Linux native.
> And the error message is definitely nicer.

Can you show the patchlet you did, and the before/after gdb
output though?  I'd be better to see that, and have any possible
follow up discussion discussion here now, rather than fractured
to a potential repost of the series.

BTW, I think that ...

+  /* Filter out requests that don't make sense during replay.  */
+  if (!record_btrace_allow_memory_access && record_btrace_is_replaying ())
+    {
+      switch (object)
+	{
+	case TARGET_OBJECT_MEMORY:
+	case TARGET_OBJECT_RAW_MEMORY:
+	case TARGET_OBJECT_STACK_MEMORY:
+	  {
+	    struct target_section *section;
+
+	    /* We do not allow writing memory in general.  */
+	    if (writebuf != NULL)
+	      throw_error (NOT_AVAILABLE_ERROR,
+			   _("This record target does not record memory."));
+
+	    /* We allow reading readonly memory.  */
+	    section = target_section_by_addr (ops, offset);
+	    if (section != NULL)
+	      {
+		/* Check if the section we found is readonly.  */
+		if ((bfd_get_section_flags (section->the_bfd_section->owner,
+					    section->the_bfd_section)
+		     & SEC_READONLY) != 0)
+		  {
+		    /* Truncate the request to fit into this section.  */
+		    len = min (len, section->endaddr - offset);
+		    break;

... here, this should return TARGET_XFER_E_IO rather than break and
falling through to the code that defers to the target beneath, as the
caller will already do that for memory transfers.

+		  }
+	      }
+
+	    throw_error (NOT_AVAILABLE_ERROR,
+			 _("This record target does not record memory."));

Then here return TARGET_XFER_E_UNAVAILABLE.

+	  }
+	}
+    }

> Should we add a new '-3' return value to be on the safe side?
> I.e. raw_memory_xfer_partial would bail out on -3 and keep
> going for -1 and -2.

I'm not seeing why we'd need that.  TARGET_XFER_E_UNAVAILABLE
is exactly designed for this use case.  It's just that
it was added recently, and we hadn't found a use outside
tracing yet.

-- 
Pedro Alves