linux-kernel - Re: [PATCH v5 2/4] dt-bindings: riscv: Add Svade and Svadu Entries

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Message-ID: <20240625-6fb62d7ea4b9f6106e27e149@orel>
Date: Tue, 25 Jun 2024 12:19:16 +0200
From: Andrew Jones <ajones@...tanamicro.com>
To: Conor Dooley <conor@...nel.org>
Cc: Alexandre Ghiti <alex@...ti.fr>, 
	Conor Dooley <conor.dooley@...rochip.com>, Anup Patel <apatel@...tanamicro.com>, 
	Yong-Xuan Wang <yongxuan.wang@...ive.com>, linux-kernel@...r.kernel.org, linux-riscv@...ts.infradead.org, 
	kvm-riscv@...ts.infradead.org, kvm@...r.kernel.org, greentime.hu@...ive.com, 
	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>, devicetree@...r.kernel.org
Subject: Re: [PATCH v5 2/4] dt-bindings: riscv: Add Svade and Svadu Entries

On Sat, Jun 22, 2024 at 01:01:30PM GMT, Conor Dooley wrote:
> On Fri, Jun 21, 2024 at 05:08:01PM +0200, Andrew Jones wrote:
> > On Fri, Jun 21, 2024 at 03:58:18PM GMT, Conor Dooley wrote:
> > > On Fri, Jun 21, 2024 at 04:52:09PM +0200, Andrew Jones wrote:
> > > > On Fri, Jun 21, 2024 at 03:04:47PM GMT, Conor Dooley wrote:
> > > > > On Fri, Jun 21, 2024 at 03:15:10PM +0200, Andrew Jones wrote:
> > > > > > On Fri, Jun 21, 2024 at 02:42:15PM GMT, Alexandre Ghiti wrote:
> > > 
> > > > > > I understand the concern; old SBI implementations will leave svadu in the
> > > > > > DT but not actually enable it. Then, since svade may not be in the DT if
> > > > > > the platform doesn't support it or it was left out on purpose, Linux will
> > > > > > only see svadu and get unexpected exceptions. This is something we could
> > > > > > force easily with QEMU and an SBI implementation which doesn't do anything
> > > > > > for svadu. I hope vendors of real platforms, which typically provide their
> > > > > > own firmware and DTs, would get this right, though, especially since Linux
> > > > > > should fail fast in their testing when they get it wrong.
> > > > > 
> > > > > I'll admit, I wasn't really thinking here about something like QEMU that
> > > > > puts extensions into the dtb before their exact meanings are decided
> > > > > upon. I almost only ever think about "real" systems, and in those cases
> > > > > I would expect that if you can update the representation of the hardware
> > > > > provided to (or by the firmware to Linux) with new properties, then updating
> > > > > the firmware itself should be possible.
> > > > > 
> > > > > Does QEMU have the this exact problem at the moment? I know it puts
> > > > > Svadu in the max cpu, but does it enable the behaviour by default, even
> > > > > without the SBI implementation asking for it?
> > > > 
> > > > Yes, because QEMU has done hardware A/D updating since it first started
> > > > supporting riscv, which means it did svadu when neither svadu nor svade
> > > > were in the DT. The "fix" for that was to ensure we have svadu and !svade
> > > > by default, which means we've perfectly realized Alexandre's concern...
> > > > We should be able to change the named cpu types that don't support svadu
> > > > to only have svade in their DTs, since that would actually be fixing those
> > > > cpu types, but we'll need to discuss how to proceed with the generic cpu
> > > > types like 'max'.
> > > 
> > > Correct me please, since I think I am misunderstanding: At the moment
> > > QEMU does A/D updating whether or not the SBI implantation asks for it,
> > > with the max CPU. The SBI implementation doesn't understand Svadu and
> > > won't strip it. The kernel will get a DT with Svadu in it, but Svadu will
> > > be enabled, so it is not a problem.
> > 
> > Oh, of course you're right! I managed to reverse things some odd number of
> > times (more than once!) in my head and ended up backwards...
> 
> I mean, I've been really confused about this whole thing the entire
> time, so ye..
> 
> Speaking of QEMU, what happens if I try to turn on svade and svadu in
> QEMU? It looks like there's some handling of it that does things
> conditionally based !svade && svade, but I couldn't tell if it would do
> what we are describing in this thread.

It'll use exception mode, but {m,h}envcfg.ADUE is still provided to allow
software to switch to hardware updating when FWFT exists. So I think we're
good to go.

Thanks,
drew