linux-kernel - Re: [RFC PATCH 0/2] Add support for SBI version to 0.2

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Message-ID: <20190916065446.GA6566@infradead.org>
Date:   Sun, 15 Sep 2019 23:54:46 -0700
From:   "hch@...radead.org" <hch@...radead.org>
To:     Palmer Dabbelt <palmer@...ive.com>
Cc:     "hch@...radead.org" <hch@...radead.org>,
        Atish Patra <Atish.Patra@....com>,
        "aou@...s.berkeley.edu" <aou@...s.berkeley.edu>,
        "alankao@...estech.com" <alankao@...estech.com>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "anup@...infault.org" <anup@...infault.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "rppt@...ux.ibm.com" <rppt@...ux.ibm.com>,
        "alexios.zavras@...el.com" <alexios.zavras@...el.com>,
        "gary@...yguo.net" <gary@...yguo.net>,
        "paul.walmsley@...ive.com" <paul.walmsley@...ive.com>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>
Subject: Re: [RFC PATCH 0/2] Add support for SBI version to 0.2

On Fri, Sep 13, 2019 at 08:54:27AM -0700, Palmer Dabbelt wrote:
> On Tue, Sep 3, 2019 at 12:38 AM hch@...radead.org <hch@...radead.org> wrote:
> 
> > On Fri, Aug 30, 2019 at 11:13:25PM +0000, Atish Patra wrote:
> > > If I understood you clearly, you want to call it legacy in the spec and
> > > just say v0.1 extensions.
> > > 
> > > The whole idea of marking them as legacy extensions to indicate that it
> > > would be obsolete in the future.
> > > 
> > > But I am not too worried about the semantics here. So I am fine with
> > > just changing the text to v0.1 if that avoids confusion.
> >
> > So my main problems is that we are lumping all the "legacy" extensions
> > together.  While some of them are simply a bad idea and shouldn't
> > really be implemented for anything new ever, others like the sfence.vma
> > and ipi ones are needed until we have hardware support to avoid them
> > and possibly forever for virtualization.
> >
> > So either we use different markers of legacy for them, or we at least
> > define new extensions that replace them at the same time.  What I
> > want to avoid is the possibіly of an implementation using the really
> > legacy bits and new extensions at the same time.
> >
> 
> Nominally we've got to replace these as well because we didn't include
> the length of the hart mask. 

Well, let's do that as part of definining the first real post-0.1
SBI then, and don't bother defining the old ones as legacy at all.

Just two different specs that don't interact except that we reserve
extension space in the new one for the old one so that one SBI spec
can implement both.