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

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Message-ID: <20220913155624.yx3dbab6x5uza2av@offworld>
Date:   Tue, 13 Sep 2022 08:56:24 -0700
From:   Davidlohr Bueso <dave@...olabs.net>
To:     Jonathan Cameron <Jonathan.Cameron@...wei.com>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>, 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 Fri, 09 Sep 2022, Jonathan Cameron wrote:

>On Thu, 8 Sep 2022 16:22:26 -0700
>Dan Williams <dan.j.williams@...el.com> wrote:
>
>> Andrew Morton wrote:
>> > On Thu, 8 Sep 2022 15:51:50 -0700 Dan Williams <dan.j.williams@...el.com> wrote:
>> >
>> > > Jonathan Cameron wrote:
>> > > > 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?
>> > >
>> > > Is "invalidate" not clear that write-back is optional? Maybe not.
>> >
>> > Yes, I'd say that "invalidate" means "dirty stuff may of may not have
>> > been written back".  Ditto for invalidate_inode_pages2().
>> >
>> > > Also, I realized that we tried to include the address range to allow for
>> > > the possibility of flushing by virtual address range, but that
>> > > overcomplicates the use. I.e. if someone issue secure erase and the
>> > > region association is not established does that mean that mean that the
>> > > cache invalidation is not needed? It could be the case that someone
>> > > disables a device, does the secure erase, and then reattaches to the
>> > > same region. The cache invalidation is needed, but at the time of the
>> > > secure erase the HPA was unknown.
>> > >
>> > > All this to say that I feel the bikeshedding will need to continue until
>> > > morale improves.
>> > >
>> > > I notice that the DMA API uses 'sync' to indicate, "make this memory
>> > > consistent/coherent for the CPU or the device", so how about an API like
>> > >
>> > >     memregion_sync_for_cpu(int res_desc)
>> > >
>> > > ...where the @res_desc would be IORES_DESC_CXL for all CXL and
>> > > IORES_DESC_PERSISTENT_MEMORY for the current nvdimm use case.
>> >
>> > "sync" is another of my pet peeves ;) In filesystem land, at least.
>> > Does it mean "start writeback and return" or does it mean "start
>> > writeback, wait for its completion then return".
>>
>> Ok, no "sync" :).
>>
>> /**
>>  * cpu_cache_invalidate_memregion - drop any CPU cached data for
>>  *     memregions described by @res_des
>>  * @res_desc: one of the IORES_DESC_* types
>>  *
>>  * Perform cache maintenance after a memory event / operation that
>>  * changes the contents of physical memory in a cache-incoherent manner.
>>  * For example, memory-device secure erase, or provisioning new CXL
>>  * regions. This routine may or may not write back any dirty contents
>>  * while performing the invalidation.
>>  *
>>  * Returns 0 on success or negative error code on a failure to perform
>>  * the cache maintenance.
>>  */
>> int cpu_cache_invalidate_memregion(int res_desc)
>
>lgtm

Likewise, and I don't see anyone else objecting so I'll go ahead and send
a new iteration.

Thanks,
Davidlohr