linux-kernel - Re: [PATCH v3 0/2] riscv: errata: thead: use riscv_nonstd_cache

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Date:   Thu, 12 Oct 2023 15:36:28 +0100
From:   Conor Dooley <conor@...nel.org>
To:     Jisheng Zhang <jszhang@...nel.org>
Cc:     Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>, Guo Ren <guoren@...nel.org>,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 0/2] riscv: errata: thead: use riscv_nonstd_cache_ops
 for CMO

On Thu, Oct 12, 2023 at 10:21:08PM +0800, Jisheng Zhang wrote:
> On Thu, Oct 12, 2023 at 10:14:54PM +0800, Jisheng Zhang wrote:
> > Previously, we use alternative mechanism to dynamically patch
> > the CMO operations for THEAD C906/C910 during boot for performance
> > reason. But as pointed out by Arnd, "there is already a significant
> > cost in accessing the invalidated cache lines afterwards, which is
> > likely going to be much higher than the cost of an indirect branch".
> > And indeed, there's no performance difference with GMAC and EMMC per
> > my test on Sipeed Lichee Pi 4A board.
> > 
> > Use riscv_nonstd_cache_ops for THEAD C906/C910 CMO to simplify
> > the alternative code, and to acchieve Arnd's goal -- "I think
> > moving the THEAD ops at the same level as all nonstandard operations
> > makes sense, but I'd still leave CMO as an explicit fast path that
> > avoids the indirect branch. This seems like the right thing to do both
> > for readability and for platforms on which the indirect branch has a
> > noticeable overhead."
> > 
> > To make bisect easy, I use two patches here: patch1 does the conversion
> > which just mimics current CMO behavior via. riscv_nonstd_cache_ops, I
> > assume no functionalities changes. patch2 uses T-HEAD PA based CMO
> > instructions so that we don't need to covert PA to VA.
> > 
> > Hi Guo,
> > 
> > I didn't use wback_inv for wback as you suggested during v1 reviewing,
> > this can be left as future optimizations.
> > 
> > Thanks
> > 
> > since v2:
> >   - collect Reviewed-by tag
> 
> Oh, I missed the tag collection, but I know maintainers are using b4 which can
> collect and apply tags automatically ;). let me know if want a new
> version.

It doesn't collect tags (AFAIU) from earlier revisions though.

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