Re: [RFC PATCH] arm64: remove CONFIG_DEBUG_ALIGN_RODATA feature

[Robin Murphy <robin.murphy@arm.com>]

On 2020-03-30 2:53 pm, Ard Biesheuvel wrote:
> On Mon, 30 Mar 2020 at 15:51, Will Deacon <will@kernel.org> wrote:
>>
>> On Sun, Mar 29, 2020 at 04:12:58PM +0200, Ard Biesheuvel wrote:
>>> When CONFIG_DEBUG_ALIGN_RODATA is enabled, kernel segments mapped with
>>> different permissions (r-x for .text, r-- for .rodata, rw- for .data,
>>> etc) are rounded up to 2 MiB so they can be mapped more efficiently.
>>> In particular, it permits the segments to be mapped using level 2
>>> block entries when using 4k pages, which is expected to result in less
>>> TLB pressure.
>>>
>>> However, the mappings for the bulk of the kernel will use level 2
>>> entries anyway, and the misaligned fringes are organized such that they
>>> can take advantage of the contiguous bit, and use far fewer level 3
>>> entries than would be needed otherwise.
>>>
>>> This makes the value of this feature dubious at best, and since it is not
>>> enabled in defconfig or in the distro configs, it does not appear to be
>>> in wide use either. So let's just remove it.
>>>
>>> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
>>> ---
>>>   arch/arm64/Kconfig.debug                  | 13 -------------
>>>   arch/arm64/include/asm/memory.h           | 12 +-----------
>>>   drivers/firmware/efi/libstub/arm64-stub.c |  8 +++-----
>>>   3 files changed, 4 insertions(+), 29 deletions(-)
>>
>> Acked-by: Will Deacon <will@kernel.org>
>>
>> But I would really like to go a step further and rip out the block mapping
>> support altogether so that we can fix non-coherent DMA aliases:
>>
>> https://lore.kernel.org/lkml/20200224194446.690816-1-hch@lst.de
>>
> 
> I'm not sure I follow - is this about mapping parts of the static
> kernel Image for non-coherent DMA?

Yikes, I hope not!

The concern there is about block entries in the linear map; I'd assume 
kernel text/data means not-linear-map, and is thus a different kettle of 
fish anyway.

Robin.