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

[Jessica Clarke]

On 2 Oct 2023, at 16:32, ron minnich <rminnich at gmail.com> wrote:
> 
> This was a very interesting read. One other thought crossed my mind,
> which is that a RISC-V implementation might make the alignment
> delegation hard-wired to always delegate to S mode. I.e, the bit might
> be WARL and always 1. For what I'm doing, this would actually be
> pretty convenient. Just want to make sure this code can accommodate
> that -- wdyt?

Such an implementation would violate the spec:

  An implementation shall not have any bits of medeleg be read-only
  one, i.e., any synchronous trap that can be delegated must support not
  being delegated.

Supporting that is thus out of scope.

Jess

> We have found lots of value in our experiments with delegating
> alignment traps to Linux -- not least because they tend to locate
> problems in the kernel :-) -- we've found issues in module loading,
> early startup (there's a needed .align2 directive for sbi secondary
> startup, AFAICT) and the timing code for misaligned load/store
> handling.
> 
> I don't know how you test this unaligned trap handling, but it might
> be worthwhile to work that out. You can test via oreboot and the
> visionfive2, save we have not figured out why SMP startup is going
> wrong, yet :-), so we're not as feature-complete as needed. But soon.
> 
> Thanks!
> 
> On Mon, Oct 2, 2023 at 5:19 AM Clément Léger <cleger at rivosinc.com> wrote:
>> 
>> 
>> 
>> On 02/10/2023 12:49, Conor Dooley wrote:
>>> On Mon, Oct 02, 2023 at 09:40:04AM +0200, Clément Léger wrote:
>>>> 
>>>> 
>>>> On 30/09/2023 11:23, Conor Dooley wrote:
>>>>> On Tue, Sep 26, 2023 at 05:03:09PM +0200, Clément Léger 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].
>>>>>> 
>>>>>> This series can be tested using the spike simulator [3] and an openSBI
>>>>>> version [4] which allows to always delegate misaligned load/store to
>>>>>> S-mode.
>>>>> 
>>>>> Some patches in this series do not build for any configs, some are
>>>>> broken for clang builds and others are broken for nommu. Please try to> build test this more thoroughly before you submit the next version.
>>>> 
>>>> Hi Conor,
>>>> 
>>>> Thanks for the feedback, I'll check that.
>>>> 
>>>>> 
>>>>> Also, AIUI, this series should be marked RFC since the SBI extension
>>>>> this relies on has not been frozen.
>>>> 
>>>> This series does not actually uses the SBI extension but provides a way
>>>> to detect if misaligned accesses are not handled by hardware nor by the
>>>> SBI. It has been reported by Ron & Daniel they they have a minimal SBI
>>>> implementation that does not handle misaligned accesses and that they
>>>> would like to make use of the PR_SET_UNALIGN feature. This is what this
>>>> series addresses (and thus does not depend on the mentioned SBI extension).
>>> 
>>> Ah, I must have misread then. Apologies.
>> 
>> No worries, maybe I should actually remove this from the cover letter to
>> avoid any confusion !
>> 
>> Clément
> 
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