Re: [PATCH v2] nd_blk: add support for "read flush" DSM flag

[Will Deacon <will.deacon@arm.com>]

On Wed, Aug 19, 2015 at 11:48:04PM +0100, Ross Zwisler wrote:
> Add support for the "read flush" _DSM flag, as outlined in the DSM spec:
> 
> http://pmem.io/documents/NVDIMM_DSM_Interface_Example.pdf
> 
> This flag tells the ND BLK driver that it needs to flush the cache lines
> associated with the aperture after the aperture is moved but before any
> new data is read.  This ensures that any stale cache lines from the
> previous contents of the aperture will be discarded from the processor
> cache, and the new data will be read properly from the DIMM.  We know
> that the cache lines are clean and will be discarded without any
> writeback because either a) the previous aperture operation was a read,
> and we never modified the contents of the aperture, or b) the previous
> aperture operation was a write and we must have written back the dirtied
> contents of the aperture to the DIMM before the I/O was completed.

Is this operation expected to be implemented as a destructive invalidation
(i.e. discarding any dirty lines from the cache) or also a writeback of any
dirtylines as part of the invalidation?

If its the former, we might want to give it a scarier name to ensure that
it doesn't grow users outside of NVDIMM scenarios, since "flush" is used
elsewhere for things like flush_dcache_page.

Will