[Bug gdb/23127] New: [AArch64] GDB cannot be used for debugging software that uses high Virtual Addresses

["robin.randhawa at arm dot com" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=23127

            Bug ID: 23127
           Summary: [AArch64] GDB cannot be used for debugging software
                    that uses high Virtual Addresses
           Product: gdb
           Version: HEAD
            Status: UNCONFIRMED
          Severity: critical
          Priority: P2
         Component: gdb
          Assignee: unassigned at sourceware dot org
          Reporter: robin.randhawa at arm dot com
  Target Milestone: ---

The patch at:

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=commit;h=a738ea1d41daeec0cccb4ab6671f4f6d53bd9e18

.. incorrectly assumes that GDB for AArch64 is only ever going to be used for
memory accesses via TTBR0. The patch ends up zeroing the top byte for high
virtual addresses that are translated via TTBR1. This makes GDB unusable for
debugging typical OS kernels etc.

I understand from some offline discussions that a key part of the problem is
that establishing the state of the top-byte-ignore control bits (TCR.TBI0 and
TCR.TBI1 for TTBR0 and TTBR1 respectively) is currently not possible with GDB.

I'm filing this bug as a matter of record in the hope that it saves someone
some bother and that it may be addressed by the experts in an architecturally
clean way.

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