Re: [PATCH 0/3 v3] [AArch64] Support tagged pointer

[Pedro Alves <palves at redhat dot com>]

On 04/11/2018 12:12 PM, Omair Javaid wrote:
> On 11 April 2018 at 15:13, Pedro Alves <palves@redhat.com <mailto:palves@redhat.com>> wrote:
> 
>     On 04/11/2018 01:15 AM, Omair Javaid wrote:
> 
>     > This patch has broken kernel debugging using kgdb and openOCD.
> 
>     OOC, can you qualify this a bit more, please?
> 
>     Does the kernel use the high bits for something?
> 
> 
> We can safely assume that top byte is 0 in case of user address space on linux because it enables tagging support but not for kernel address space.
> 
> According to linux memory layout of AArch64 given here: https://www.kernel.org/doc/Documentation/arm64/memory.txt
> 
> "User addresses have bits 63:48 set to 0 while the kernel addresses have
                                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> the same bits set to 1. TTBRx selection is given by bit 63 of the
^^^^^^^^^^^^^^^^^^^^^^^^^
> virtual address."

Ah, that's clear as day now.

> 
> According to kernel document on tagged pointer support in AArch64 given here: https://www.kernel.org/doc/Documentation/arm64/tagged-pointers.txt
> 
> The kernel configures the translation tables so that translations made
> via TTBR0 (i.e. userspace mappings) have the top byte (bits 63:56) of
> the virtual address ignored by the translation hardware. This frees up
> this byte for application use.
> 
> With set_gdbarch_significant_addr_bit applied to aarch64-tdep following happens when gdb tries reading kernel address space memory:
> 
> query the 0xffffffc000092698 memory data, GDB sent "m00ffffc000092698,4" instead of "mffffffc000092698,4"
> 
OK, that makes a lot more sense now.  The above is the perfect
info to be included in a git commit log.  Want to submit a patch?

Thanks,
Pedro Alves