linux-kernel - Re: [PATCH -next] memregion: Add arch_flush

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Date:   Thu, 8 Sep 2022 14:13:19 +0100
From:   Jonathan Cameron <Jonathan.Cameron@...wei.com>
To:     Dan Williams <dan.j.williams@...el.com>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Davidlohr Bueso <dave@...olabs.net>, <x86@...nel.org>,
        <nvdimm@...ts.linux.dev>, <linux-cxl@...r.kernel.org>,
        <peterz@...radead.org>, <bp@...en8.de>, <dave.jiang@...el.com>,
        <vishal.l.verma@...el.com>, <ira.weiny@...el.com>,
        <a.manzanares@...sung.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -next] memregion: Add arch_flush_memregion() interface

On Wed, 7 Sep 2022 18:07:31 -0700
Dan Williams <dan.j.williams@...el.com> wrote:

> Andrew Morton wrote:
> > I really dislike the term "flush".  Sometimes it means writeback,
> > sometimes it means invalidate.  Perhaps at other times it means
> > both.
> > 
> > Can we please be very clear in comments and changelogs about exactly
> > what this "flush" does.   With bonus points for being more specific in the 
> > function naming?
> >   
> 
> That's a good point, "flush" has been cargo-culted along in Linux's
> cache management APIs to mean write-back-and-invalidate. In this case I
> think this API is purely about invalidate. It just so happens that x86
> has not historically had a global invalidate instruction readily
> available which leads to the overuse of wbinvd.
> 
> It would be nice to make clear that this API is purely about
> invalidating any data cached for a physical address impacted by address
> space management event (secure erase / new region provision). Write-back
> is an unnecessary side-effect.
> 
> So how about:
> 
> s/arch_flush_memregion/cpu_cache_invalidate_memregion/?

Want to indicate it 'might' write back perhaps?
So could be invalidate or clean and invalidate (using arm ARM terms just to add
to the confusion ;)

Feels like there will be potential race conditions where that matters as we might
force stale data to be written back.

Perhaps a comment is enough for that. Anyone have the "famous last words" feeling?

Jonathan