On Tue, Jul 14, 2020 at 12:42:11PM -0700, Dave Hansen wrote:

On 7/14/20 12:29 PM, Peter Zijlstra wrote:

quoted

On Tue, Jul 14, 2020 at 12:06:16PM -0700, Ira Weiny wrote:

quoted

On Tue, Jul 14, 2020 at 10:44:51AM +0200, Peter Zijlstra wrote:

quoted

So, if I followed along correctly, you're proposing to do a WRMSR per
k{,un}map{_atomic}(), sounds like excellent performance all-round :-(

Only to pages which have this additional protection, ie not DRAM.

User mappings of this memory is not affected (would be covered by User PKeys if
desired).  User mappings to persistent memory are the primary use case and the
performant path.

Because performance to non-volatile memory doesn't matter? I think Dave
has a better answer here ...

So, these WRMSRs are less evil than normal.  They're architecturally
non-serializing instructions,

Excellent, that should make these a fair bit faster than regular MSRs.

But, either way, this *will* make accessing PMEM more expensive from the
kernel.  No escaping that. 

There's no free lunch, it's just that regular MSRs are fairly horrible.