linux-kernel - Re: [RISC-V] [tech-j-ext] [RFC PATCH 5/9] riscv: Split per-CPU and per-thread envcfg bits

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Date: Wed, 20 Mar 2024 08:06:20 +0000
From: Conor Dooley <conor.dooley@...rochip.com>
To: Samuel Holland <samuel.holland@...ive.com>
CC: Deepak Gupta <debug@...osinc.com>, Palmer Dabbelt <palmer@...belt.com>,
	<linux-riscv@...ts.infradead.org>, <devicetree@...r.kernel.org>, Catalin
 Marinas <catalin.marinas@....com>, <linux-kernel@...r.kernel.org>, Conor
 Dooley <conor@...nel.org>, <kasan-dev@...glegroups.com>, Evgenii Stepanov
	<eugenis@...gle.com>, Krzysztof Kozlowski
	<krzysztof.kozlowski+dt@...aro.org>, Rob Herring <robh+dt@...nel.org>, Andrew
 Jones <ajones@...tanamicro.com>, Guo Ren <guoren@...nel.org>, Heiko Stuebner
	<heiko@...ech.de>, Paul Walmsley <paul.walmsley@...ive.com>
Subject: Re: [RISC-V] [tech-j-ext] [RFC PATCH 5/9] riscv: Split per-CPU and
 per-thread envcfg bits

On Tue, Mar 19, 2024 at 09:20:59PM -0500, Samuel Holland wrote:
> On 2024-03-19 6:55 PM, Deepak Gupta wrote:
> > On Tue, Mar 19, 2024 at 2:59 PM Samuel Holland via lists.riscv.org
> > <samuel.holland=sifive.com@...ts.riscv.org> wrote:
> >>
> >> Some envcfg bits need to be controlled on a per-thread basis, such as
> >> the pointer masking mode. However, the envcfg CSR value cannot simply be
> >> stored in struct thread_struct, because some hardware may implement a
> >> different subset of envcfg CSR bits is across CPUs. As a result, we need
> >> to combine the per-CPU and per-thread bits whenever we switch threads.
> >>
> > 
> > Why not do something like this
> > 
> > diff --git a/arch/riscv/include/asm/csr.h b/arch/riscv/include/asm/csr.h
> > index b3400517b0a9..01ba87954da2 100644
> > --- a/arch/riscv/include/asm/csr.h
> > +++ b/arch/riscv/include/asm/csr.h
> > @@ -202,6 +202,8 @@
> >  #define ENVCFG_CBIE_FLUSH              _AC(0x1, UL)
> >  #define ENVCFG_CBIE_INV                        _AC(0x3, UL)
> >  #define ENVCFG_FIOM                    _AC(0x1, UL)
> > +/* by default all threads should be able to zero cache */
> > +#define ENVCFG_BASE                    ENVCFG_CBZE
> 
> Linux does not assume Sstrict, so without Zicboz being present in DT/ACPI, we
> have no idea what the CBZE bit does--there's no guarantee it has the standard
> meaning--so it's not safe to set the bit unconditionally. If that policy
> changes, we could definitely simplify the code.

The wording for that "extension", if two lines in the profiles doc makes
something an extension is:
"No non-conforming extensions are present. Attempts to execute unimplemented
opcodes or access unimplemented CSRs in the standard or reserved encoding
spaces raises an illegal instruction exception that results in a contained
trap to the supervisor-mode trap handler."

I know we have had new extensions come along and mark previously fair
game interrupts for vendors as out of bounds. I wonder if there's a risk
of that happening with CSRs or opcodes too (or maybe it has happened and
I cannot recall).

Going back to the interrupts - is the Andes PMU non-conforming because
it uses an interrupt that was declared as vendor usable but is now part
of the standard space because of AIA? If it is, then the meaning of
Sstrict could vary wildly based on the set of extensions (and their
versions for specs). That sounds like a lot of fun.

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