linux-kernel - Re: [PATCH v6 2/4] dt-bindings: riscv: Add Svade and Svadu Entries

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Date: Sun, 30 Jun 2024 15:09:33 +0100
From: Conor Dooley <conor@...nel.org>
To: Jessica Clarke <jrtc27@...c27.com>
Cc: Yong-Xuan Wang <yongxuan.wang@...ive.com>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-riscv <linux-riscv@...ts.infradead.org>,
	kvm-riscv@...ts.infradead.org, kvm@...r.kernel.org,
	Greentime Hu <greentime.hu@...ive.com>,
	Vincent Chen <vincent.chen@...ive.com>,
	Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Paul Walmsley <paul.walmsley@...ive.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>,
	"open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" <devicetree@...r.kernel.org>
Subject: Re: [PATCH v6 2/4] dt-bindings: riscv: Add Svade and Svadu Entries

On Sat, Jun 29, 2024 at 02:09:34PM +0100, Jessica Clarke wrote:
> On 28 Jun 2024, at 17:19, Conor Dooley <conor@...nel.org> 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@...ive.com>
> >> ---
> >> .../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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