On Tue, Jan 30, 2018 at 3:49 PM, Greentime Hu [off-list ref] wrote:

Hi, Arnd:

2018-01-30 21:33 GMT+08:00 Arnd Bergmann [off-list ref]:

quoted

On Tue, Jan 30, 2018 at 11:01 AM, Vincent Chen [off-list ref] wrote:

quoted

2018-01-24 19:10 GMT+08:00 Arnd Bergmann [off-list ref]:

quoted

On Wed, Jan 24, 2018 at 12:09 PM, Arnd Bergmann [off-list ref] wrote:

quoted

On Wed, Jan 24, 2018 at 11:53 AM, Vincent Chen [off-list ref] wrote:

quoted

2018-01-18 18:14 GMT+08:00 Arnd Bergmann [off-list ref]:

quoted

Ok. I still wonder about the kernel part of this though: is it a good idea
for user space to configure whether the kernel does unaligned
accesses? I would think that the kernel should just be fixed in such
a case.

To clarify: I'm asking only about unaligned accesses from kernel code itself,
which is generally considered a bug when
CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS is disabled.

      Arnd

Thanks for your comments.

For performance, we decide always disable
CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS even if hardware supports
unaligned accessing. Therefore, I will remove kernel unaligned accessing from
nds32/mm/alignment.c. In other words, alignment.c only addresses unaligned
accessing for user space.

I'm not really following that logic, let's go through that again so I understand
the situation better.

CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS should be set if and
only if you have a CPU that does not need to trap on unaligned accesses.

What are the hardware capabilities on nds32? Do you have all three
categories:

a) some CPUs that always trap on unaligned access
b) some CPUs that never trap on unaligned access
c) some CPUs that can be configured to either trap or not trap by
    the kernel?

We have type a and c.
We use CONFIG_ALIGNMENT_TRAP for a and
CONFIG_HW_SUPPORT_UNALIGNMENT_ACCESS for c.

Ok, got it.

Since unaligned access in kernel code itself should be considered as a
bug, we will remove the emulation code to handle the kernel code
unaligned accessed case.
We think CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS and
CONFIG_HW_SUPPORT_UNALIGNMENT_ACCESS have different purposes because
it will still be more efficient to access by byte even if hardware
support unaligned access.
CONFIG_HW_SUPPORT_UNALIGNMENT_ACCESS is used to prevent generating
unaligned access exception.

Hmm, this is a bit tricky. Most architectures actually assume that those
two are the same, and nothing else has a
HW_SUPPORT_UNALIGNMENT_ACCESS option.

We do actually have a related problem on 32-bit ARM, where the current
generation of processors (ARMv6 and higher) support unaligned
accesses for almost all instructions with the exception of those
instructions that operate on multiple memory locations (ldm/stm
and ldrd/strd). We can control the use of those instructions in inline
assembler, and gcc never uses them when it knows that a pointer
is unaligned, but when CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
is set, the kernel sometimes intentionally contains code sequences
that lead the compiler to believe that a variable is aligned when it
is not, so we end up needing a trap handler here.

We might at some point want to clean this up by going through
all uses of CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
and changing them in a way that leads to better results on both
arm32 and nds32.

       Arnd