Re: PCIe MSI-X interrupts

EricP <ThatWouldBeTelling@thevillage.com> wrote:

Michael S wrote:

On Thu, 27 Jun 2024 01:47:49 +0000
mitchalsup@aol.com (MitchAlsup1) wrote:
 

Exactly what are you intending to mean from "single-copy atomic
accesses" ??

 
It sounds as a politically correct way of saying "default memory
ordering of ARMv8.1-A and later".
I.e. weaker than x86-64 and SPARC TSO, but stronger than Itanium.
Probably stronger than POWER, but I am not sure if POWER ever had memory
ordering model formalized.
 

 
Multi-copy atomic is ARM's name for a write-update coherence protocol
as it allows each cache to have its own copy of a single memory location.

The terminology is not Arm's, it comes from

William W. Collier. 1992. Reasoning about parallel architectures.
Prentice Hall, Englewood Cliffs.

Single-copy atomic is their name for a write-invalidate protocol
as it ensures that there is one value for each memory location.
 
Originally ARM's weak cache coherence protocol spec, like Alpha,
did not explicitly exclude multi-copy atomic so software designers had
to consider all the extra race conditions a write-update implementation
might allow. But this was wasted extra effort because no one implements
a write-update protocol, just write-invalidate.
Eventually ARM specified that it was single-copy atomic (write-invalidate).

And it's now multi-copy atomic, thank goodness.

Andrew.