On Sat, Jun 29, 2024 at 02:09:34PM +0100, Jessica Clarke wrote:
> On 28 Jun 2024, at 17:19, Conor Dooley <conor@xxxxxxxxxx> wrote:
> >
> > On Fri, Jun 28, 2024 at 05:37:06PM +0800, Yong-Xuan Wang wrote:
> >> Add entries for the Svade and Svadu extensions to the riscv,isa-extensions
> >> property.
> >>
> >> Signed-off-by: Yong-Xuan Wang <yongxuan.wang@xxxxxxxxxx>
> >> ---
> >> .../devicetree/bindings/riscv/extensions.yaml | 28 +++++++++++++++++++
> >> 1 file changed, 28 insertions(+)
> >>
> >> diff --git a/Documentation/devicetree/bindings/riscv/extensions.yaml b/Documentation/devicetree/bindings/riscv/extensions.yaml
> >> index 468c646247aa..c3d053ce7783 100644
> >> --- a/Documentation/devicetree/bindings/riscv/extensions.yaml
> >> +++ b/Documentation/devicetree/bindings/riscv/extensions.yaml
> >> @@ -153,6 +153,34 @@ properties:
> >> ratified at commit 3f9ed34 ("Add ability to manually trigger
> >> workflow. (#2)") of riscv-time-compare.
> >>
> >> + - const: svade
> >> + description: |
> >> + The standard Svade supervisor-level extension for SW-managed PTE A/D
> >> + bit updates as ratified in the 20240213 version of the privileged
> >> + ISA specification.
> >> +
> >> + Both Svade and Svadu extensions control the hardware behavior when
> >> + the PTE A/D bits need to be set. The default behavior for the four
> >> + possible combinations of these extensions in the device tree are:
> >> + 1) Neither Svade nor Svadu present in DT =>
> >
> >> It is technically
> >> + unknown whether the platform uses Svade or Svadu. Supervisor may
> >> + assume Svade to be present and enabled or it can discover based
> >> + on mvendorid, marchid, and mimpid.
> >
> > I would just write "for backwards compatibility, if neither Svade nor
> > Svadu appear in the devicetree the supervisor may assume Svade to be
> > present and enabled". If there are systems that this behaviour causes
> > problems for, we can deal with them iff they appear. I don't think
> > looking at m*id would be sufficient here anyway, since the firmware can
> > have an impact. I'd just drop that part entirely.
>
> Older QEMU falls into that category, as do Bluespec’s soft-cores (which
> ours are derived from at Cambridge). I feel that, in reality, one
> should be prepared to handle both trapping and atomic updates if
> writing an OS that aims to support case 1.
I guess that is actually what we should put in then, to use an
approximation of your wording, something like
Neither Svade nor Svadu present in DT => Supervisor software should be
prepared to handle either hardware updating of the PTE A/D bits or page
faults when they need updated
?
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