Date: Tue, 4 Jun 2024 15:26:48 +0100
From: Jonathan Cameron <Jonathan.Cameron@...wei.com>
To: James Morse <james.morse@....com>
CC: Dan Williams <dan.j.williams@...el.com>, Dongsheng Yang
	<dongsheng.yang@...ystack.cn>, Gregory Price <gregory.price@...verge.com>,
	John Groves <John@...ves.net>, <axboe@...nel.dk>,
	<linux-block@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<linux-cxl@...r.kernel.org>, <nvdimm@...ts.linux.dev>, Mark Rutland
	<mark.rutland@....com>
Subject: Re: [PATCH RFC 0/7] block: Introduce CBD (CXL Block Device)

On Mon, 3 Jun 2024 18:28:51 +0100
James Morse <james.morse@....com> wrote:

> Hi guys,
> 
> On 03/06/2024 13:48, Jonathan Cameron wrote:
> > On Fri, 31 May 2024 20:22:42 -0700
> > Dan Williams <dan.j.williams@...el.com> wrote:  
> >> Jonathan Cameron wrote:  
> >>> On Thu, 30 May 2024 14:59:38 +0800
> >>> Dongsheng Yang <dongsheng.yang@...ystack.cn> wrote:  
> >>>> 在 2024/5/29 星期三 下午 11:25, Gregory Price 写道:    
> >>>>> It's not just a CXL spec issue, though that is part of it. I think the
> >>>>> CXL spec would have to expose some form of puncturing flush, and this
> >>>>> makes the assumption that such a flush doesn't cause some kind of
> >>>>> race/deadlock issue.  Certainly this needs to be discussed.
> >>>>>
> >>>>> However, consider that the upstream processor actually has to generate
> >>>>> this flush.  This means adding the flush to existing coherence protocols,
> >>>>> or at the very least a new instruction to generate the flush explicitly.
> >>>>> The latter seems more likely than the former.
> >>>>>
> >>>>> This flush would need to ensure the data is forced out of the local WPQ
> >>>>> AND all WPQs south of the PCIE complex - because what you really want to
> >>>>> know is that the data has actually made it back to a place where remote
> >>>>> viewers are capable of percieving the change.
> >>>>>
> >>>>> So this means:
> >>>>> 1) Spec revision with puncturing flush
> >>>>> 2) Buy-in from CPU vendors to generate such a flush
> >>>>> 3) A new instruction added to the architecture.
> >>>>>
> >>>>> Call me in a decade or so.
> >>>>>
> >>>>>
> >>>>> But really, I think it likely we see hardware-coherence well before this.
> >>>>> For this reason, I have become skeptical of all but a few memory sharing
> >>>>> use cases that depend on software-controlled cache-coherency.      
> >>>>
> >>>> Hi Gregory,
> >>>>
> >>>> 	From my understanding, we actually has the same idea here. What I am 
> >>>> saying is that we need SPEC to consider this issue, meaning we need to 
> >>>> describe how the entire software-coherency mechanism operates, which 
> >>>> includes the necessary hardware support. Additionally, I agree that if 
> >>>> software-coherency also requires hardware support, it seems that 
> >>>> hardware-coherency is the better path.    
> >>>>>
> >>>>> There are some (FAMFS, for example). The coherence state of these
> >>>>> systems tend to be less volatile (e.g. mappings are read-only), or
> >>>>> they have inherent design limitations (cacheline-sized message passing
> >>>>> via write-ahead logging only).      
> >>>>
> >>>> Can you explain more about this? I understand that if the reader in the 
> >>>> writer-reader model is using a readonly mapping, the interaction will be 
> >>>> much simpler. However, after the writer writes data, if we don't have a 
> >>>> mechanism to flush and invalidate puncturing all caches, how can the 
> >>>> readonly reader access the new data?    
> >>>
> >>> There is a mechanism for doing coarse grained flushing that is known to
> >>> work on some architectures. Look at cpu_cache_invalidate_memregion().
> >>> On intel/x86 it's wbinvd_on_all_cpu_cpus()    
> >>
> >> There is no guarantee on x86 that after cpu_cache_invalidate_memregion()
> >> that a remote shared memory consumer can be assured to see the writes
> >> from that event.  
> > 
> > I was wondering about that after I wrote this...  I guess it guarantees
> > we won't get a late landing write or is that not even true?
> > 
> > So if we remove memory, then added fresh memory again quickly enough
> > can we get a left over write showing up?  I guess that doesn't matter as
> > the kernel will chase it with a memset(0) anyway and that will be ordered
> > as to the same address.
> > 
> > However we won't be able to elide that zeroing even if we know the device
> > did it which is makes some operations the device might support rather
> > pointless :(  
> 
> >>> on arm64 it's a PSCI firmware call CLEAN_INV_MEMREGION (there is a
> >>> public alpha specification for PSCI 1.3 with that defined but we
> >>> don't yet have kernel code.)    
> 
> I have an RFC for that - but I haven't had time to update and re-test it.

If it's useful, I might either be able to find time to take that forwards
(or get someone else to do it).

Let me know if that would be helpful; I'd love to add this to the list
of things I can forget about because it just works for kernel
(and hence is a problem for the firmware and uarch folk).

> 
> If you need this, and have a platform where it can be implemented, please get in touch
> with the people that look after the specs to move it along from alpha.
> 
> 
> >> That punches visibility through CXL shared memory devices?  
> 
> > It's a draft spec and Mark + James in +CC can hopefully confirm.
> > It does say
> > "Cleans and invalidates all caches, including system caches".
> > which I'd read as meaning it should but good to confirm.  
> 
> It's intended to remove any cached entries - including lines in what the arm-arm calls
> "invisible" system caches, which typically only platform firmware can touch. The next
> access should have to go all the way to the media. (I don't know enough about CXL to say
> what a remote shared memory consumer observes)

If it's out of the host bridge buffers (and known to have succeeded in write back) which I
think the host should know, I believe what happens next is a device implementer problem.
Hopefully anyone designing a device that does memory sharing has built that part right.

> 
> Without it, all we have are the by-VA operations which are painfully slow for large
> regions, and insufficient for system caches.
> 
> As with all those firmware interfaces - its for the platform implementer to wire up
> whatever is necessary to remove cached content for the specified range. Just because there
> is an (alpha!) spec doesn't mean it can be supported efficiently by a particular platform.
> 
> 
> >>> These are very big hammers and so unsuited for anything fine grained.  
> 
> You forgot really ugly too!

I was being polite :)

> 
> 
> >>> In the extreme end of possible implementations they briefly stop all
> >>> CPUs and clean and invalidate all caches of all types. So not suited
> >>> to anything fine grained, but may be acceptable for a rare setup event,
> >>> particularly if the main job of the writing host is to fill that memory
> >>> for lots of other hosts to use.
> >>>
> >>> At least the ARM one takes a range so allows for a less painful
> >>> implementation.   
> 
> That is to allow some ranges to fail. (e.g. you can do this to the CXL windows, but not
> the regular DRAM).
> 
> On the less painful implementation, arm's interconnect has a gadget that does "Address
> based flush" which could be used here. I'd hope platforms with that don't need to
> interrupt all CPUs - but it depends on what else needs to be done.
> 
> 
> >>> I'm assuming we'll see new architecture over time
> >>> but this is a different (and potentially easier) problem space
> >>> to what you need.    
> >>
> >> cpu_cache_invalidate_memregion() is only about making sure local CPU
> >> sees new contents after an DPA:HPA remap event. I hope CPUs are able to
> >> get away from that responsibility long term when / if future memory
> >> expanders just issue back-invalidate automatically when the HDM decoder
> >> configuration changes.  
> > 
> > I would love that to be the way things go, but I fear the overheads of
> > doing that on the protocol means people will want the option of the painful
> > approach.  
> 
> 
> 
> Thanks,
> 
> James

Thanks for the info,

Jonathan

> 

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