Re: [PATCH 09/12] riscv: add RISC-V Svpbmt extension support

From: "Heiko Stübner" <heiko@sntech.de>
To: Christoph Hellwig <hch@lst.de>,
	Philipp Tomsich <philipp.tomsich@vrull.eu>
Cc: palmer@dabbelt.com, paul.walmsley@sifive.com,
	aou@eecs.berkeley.edu, linux-riscv@lists.infradead.org,
	linux-kernel@vger.kernel.org, wefu@redhat.com,
	liush@allwinnertech.com, guoren@kernel.org,
	atishp@atishpatra.org, anup@brainfault.org, drew@beagleboard.org,
	arnd@arndb.de, wens@csie.org, maxime@cerno.tech,
	gfavor@ventanamicro.com, andrea.mondelli@huawei.com,
	behrensj@mit.edu, xinhaoqu@huawei.com, mick@ics.forth.gr,
	allen.baum@esperantotech.com, jscheid@ventanamicro.com,
	rtrauben@gmail.com, samuel@sholland.org, cmuellner@linux.com,
	Wei Wu <lazyparser@gmail.com>, Daniel Lustig <dlustig@nvidia.com>,
	Bill Huffman <huffman@cadence.com>
Subject: Re: [PATCH 09/12] riscv: add RISC-V Svpbmt extension support
Date: Mon, 16 May 2022 12:30:55 +0200	[thread overview]
Message-ID: <4710483.GXAFRqVoOG@diego> (raw)
In-Reply-To: <CAAeLtUA1FWQRot6=zHXWBa3YnfxuhL2TW-a-TNomDeUFPrrz1A@mail.gmail.com>

Am Montag, 16. Mai 2022, 11:09:12 CEST schrieb Philipp Tomsich:
> On Mon, 16 May 2022 at 08:11, Christoph Hellwig <hch@lst.de> wrote:
> >
> > > +config RISCV_ISA_SVPBMT
> > > +     bool "SVPBMT extension support"
> >
> > I don't think this prompt is very useful as it doesn't describe
> > what it does.  But do we even want people to disable it as it is
> > really essentially for a fully functioning kernel and a pity that
> > it took RISC-V so long to get there?
> 
> Given that RISC-V is (in some ways) an ISA construction set, there
> will be valid use cases for embedded users to disable this (e.g. if
> they have their own non-standard way to configure these).  So while
> kernels for binary distributions (and desktop, server, or
> general-purpose embedded) will always enable these, I would fully
> expect some users to want to turn these off.

Also, enabling the SVPBMT extension will pull in the alternative-patching
as well of course, and having a way to disable that was a review-request
a version in the past.

> @Heiko: I would request that we have a longer help text on this, which
> explains what it is and ends with the usual "When in doubt, say Y."

ok, will do

> > > +     depends on 64BIT && MMU
> > > +     select RISCV_ALTERNATIVE
> > > +     default y
> > > +     help
> > > +        Adds support to dynamically detect the presence of the SVPBMT extension
> >
> > overly long line here.

will fix together with Philipp's help-text wish

> > > index 5f1046e82d9f..dbfcd9b72bd8 100644
> > > --- a/arch/riscv/include/asm/errata_list.h
> > > +++ b/arch/riscv/include/asm/errata_list.h
> > > @@ -14,6 +14,9 @@
> > >  #define      ERRATA_SIFIVE_NUMBER 2
> > >  #endif
> > >
> > > +#define      CPUFEATURE_SVPBMT 0
> > > +#define      CPUFEATURE_NUMBER 1
> >
> > is errata_list.h really the right place for architectural features?

That probably stems from the alternatives being exclusively used
for erratas in the past.

I guess making this "alternative-list.h" might be a better naming?
Or are there even better suggestions?

> > Otherwise looks good:
> >
> > Reviewed-by: Christoph Hellwig <hch@lst.de>

Thanks
Heiko

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