Re: [PATCH RFC 0/7] block: Introduce CBD (CXL Block Device)

[Dongsheng Yang]

在 2024/5/31 星期五 下午 10:23, Gregory Price 写道:
> On Thu, May 30, 2024 at 02:59:38PM +0800, Dongsheng Yang wrote:
> > 在 2024/5/29 星期三 下午 11:25, Gregory Price 写道:
> > > 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?
>
> This is exactly right, so the coherence/correctness of the data needs to
> be enforced in some other way.
>
> Generally speaking, the WPQs will *eventually* get flushed.  As such,
> the memory will *eventually* become coherent.  So if you set up the
> following pattern, you will end up with an "eventually coherent" system


Yes, it is "eventually coherent" if "NO CLEAN WRITEBACK" bit in both 
CSDS and DVSEC is set.
> 1) Writer instantiates the memory to be used
> 2) Writer calculates and records a checksum of that data into memory
> 3) Writer invalidates everything
> 4) Reader maps the memory
> 5) Reader reads the checksum and calculates the checksum of the data
>     a) if the checksums match, the data is coherent
>     b) if they don't, we must wait longer for the queues to flush

Yes, the checksum was mentioned by John, it is used in FAMFS/pcq_lib.c, 
pcq use sequence and checksum in consumer to make sure data consistency.

I think it's a good idea and was planning to introduce it into cbd, of 
coures it should be optional for cbd, as cbd current only supports
hardware-consistency usage. it can be an option to do data verification.

Thanx
> This is just one example of a system design which enforces coherence by
> placing the limitation on the system that the data will never change
> once it becomes coherent.
>
> Whatever the case, regardless of the scheme you come up with, you will
> end up with a system where the data must be inspected and validated
> before it can be used.  This has the limiting factor of performance:
> throughput will be limited by how fast you can validate the data.
>
> ~Gregory
> .