linux-kernel - Re: [PATCH v6 00/14] riscv: support for Svpbmt and D1 memory types

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Date:   Thu, 10 Feb 2022 01:49:19 +0800
From:   Jisheng Zhang <jszhang@...nel.org>
To:     Heiko Stuebner <heiko@...ech.de>
Cc:     palmer@...belt.com, paul.walmsley@...ive.com,
        aou@...s.berkeley.edu, linux-riscv@...ts.infradead.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        robh+dt@...nel.org, wefu@...hat.com, liush@...winnertech.com,
        guoren@...nel.org, atishp@...shpatra.org, anup@...infault.org,
        drew@...gleboard.org, hch@....de, arnd@...db.de, wens@...e.org,
        maxime@...no.tech, gfavor@...tanamicro.com,
        andrea.mondelli@...wei.com, behrensj@....edu, xinhaoqu@...wei.com,
        huffman@...ence.com, mick@....forth.gr,
        allen.baum@...erantotech.com, jscheid@...tanamicro.com,
        rtrauben@...il.com, samuel@...lland.org, cmuellner@...ux.com,
        philipp.tomsich@...ll.eu
Subject: Re: [PATCH v6 00/14] riscv: support for Svpbmt and D1 memory types

On Wed, Feb 09, 2022 at 01:37:46PM +0100, Heiko Stuebner wrote:
> Svpbmt is an extension defining "Supervisor-mode: page-based memory types"
> for things like non-cacheable pages or I/O memory pages.
> 
> 
> So this is my 2nd try at implementing Svpbmt (and the diverging D1 memory
> types) using the alternatives framework.
> 
> This includes a number of changes to the alternatives mechanism itself.
> The biggest one being the move to a more central location, as I expect
> in the future, nearly every chip needing some sort of patching, be it
> either for erratas or for optional features (svpbmt or others).
> 
> The dt-binding for svpbmt itself is of course not finished and is still
> using the binding introduced in previous versions, as where to put
> a svpbmt-property in the devicetree is still under dicussion.
> Atish seems to be working on a framework for extensions [0],
> 
> The series also introduces support for the memory types of the D1
> which are implemented differently to svpbmt. But when patching anyway
> it's pretty clean to add the D1 variant via ALTERNATIVE_2 to the same
> location.
> 
> The only slightly bigger difference is that the "normal" type is not 0
> as with svpbmt, so kernel patches for this PMA type need to be applied
> even before the MMU is brought up, so the series introduces a separate
> stage for that.
> 
> 
> In theory this series is 3 parts:
> - sbi cache-flush / null-ptr
> - alternatives improvements
> - svpbmt+d1
> 
> So expecially patches from the first 2 areas could be applied when
> deemed ready, I just thought to keep it together to show-case where
> the end-goal is and not requiring jumping between different series.
> 
> 
> The sbi cache-flush patch is based on Atish's sparse-hartid patch [1],
> as it touches a similar area in mm/cacheflush.c
> 
> 
> I picked the recipient list from the previous version, hopefully
> I didn't forget anybody.
> 
> changes in v6:
> - rebase onto 5.17-rc1
> - handle sbi null-ptr differently
> - improve commit messages
> - use riscv,mmu as property name
> 
> changes in v5:
> - move to use alternatives for runtime-patching

Hi,

another choice is using static key mechanism. Pros: no need to coding
in asm, all in c.

To support new arch features, I see other arch sometimes use static
key, sometimes use alternative mechanism, so one question here would
be which mechanism is better? Any guide?

Thanks in advance