Re: [PATCH v1] block/nvme: introduce PMR support from NVMe 1.4 spec

[Stefan Hajnoczi]

On Fri, Feb 21, 2020 at 3:36 PM Andrzej Jakowski
<address@hidden> wrote:
> On 2/21/20 6:45 AM, Stefan Hajnoczi wrote:
> > Why is msync(2) done on memory loads instead of stores?
>
> This is my interpretation of NVMe spec wording with regards to PMRWBM field
> which says:
>
> "The completion of a memory read from any Persistent
> Memory Region address ensures that all prior writes to the
> Persistent Memory Region have completed and are
> persistent."

Thanks, I haven't read the PMR section of the spec :).

A synchronous operation is bad for virtualization performance.  While
the sync may be a cheap operation in hardware, it can be arbitrarily
expensive with msync(2).  The vCPU will be stuck until msync(2)
completes on the host.

It's also a strange design choice since performance will suffer when
an unrelated read has to wait for writes to complete.  This is
especially problematic for multi-threaded applications or multi-core
systems where I guess this case is hit frequently.  Maybe it's so
cheap in hardware that it doesn't matter?  But then why didn't NVDIMM
use this mechanism?

If anyone knows the answer I'd be interested in learning.  But this
isn't a criticism of the patch - of course it needs to implement the
hardware spec and we can't change it.

Stefan