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

[ron minnich]

While it is true that it violates the spec today, given the fluidity
of the spec of the last 10 years, I'm not sure that matters :-)

Anyway, that's out of scope for this discussion, though I appreciate
your clarification. I'll bring it up elsewhere.

Clement points out that this series would work fine if that bit were
hardwired to 1, which is all I care about.

thanks

On Mon, Oct 2, 2023 at 4:23 PM Jessica Clarke <jrtc27@xxxxxxxxxx> wrote:
>
> On 2 Oct 2023, at 16:32, ron minnich <rminnich@xxxxxxxxx> 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@xxxxxxxxxxxx> 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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