[PATCH 0/7] Add support to handle misaligned accesses in S-mode

[Clément Léger]

On 26/09/2023 23:43, Evan Green wrote:
> On Tue, Sep 26, 2023 at 8:03 AM Clément Léger <cleger at rivosinc.com> wrote:
>>
>> Since commit 61cadb9 ("Provide new description of misaligned load/store
>> behavior compatible with privileged architecture.") in the RISC-V ISA
>> manual, it is stated that misaligned load/store might not be supported.
>> However, the RISC-V kernel uABI describes that misaligned accesses are
>> supported. In order to support that, this series adds support for S-mode
>> handling of misaligned accesses as well support for prctl(PR_UNALIGN).
>>
>> Handling misaligned access in kernel allows for a finer grain control
>> of the misaligned accesses behavior, and thanks to the prctl call, can
>> allow disabling misaligned access emulation to generate SIGBUS. User
>> space can then optimize its software by removing such access based on
>> SIGBUS generation.
>>
>> Currently, this series is useful for people that uses a SBI that does
>> not handled misaligned traps. In a near future, this series will make
>> use a SBI extension [1] allowing to request delegation of the
>> misaligned load/store traps to the S-mode software. This extension has
>> been submitted for review to the riscv tech-prs group. An OpenSBI
>> implementation for this spec is available at [2].
> 
> For my own education, how does the new SBI call behave with respect to
> multiple harts? Does a call to change a feature perform that change
> across all harts, or just the hart the SBI call was made on? If the
> answer is "all harts", what if not all harts are exactly the same, and
> some can enable the feature switch while others cannot? Also if the
> answer is "all harts", does it also apply to hotplugged cpus, which
> may not have even existed at boot time?

Depending on the feature, they can be either global (all harts) or
local (calling hart). The medeleg register is per hart and thus
misaligned load/store delegation for S-mode is also per hart.


> 
> What happens if a hart goes through a context loss event, like
> suspend/resume? Is the setting expected to be sticky, or is the kernel
> expected to replay these calls?

That is a good question that we did not actually clarified yet. Thanks
for raising it !

Clément

> 
> -Evan