Re: [PATCH RFC] hw/cxl: type 3 devices can now present volatile or persistent memory

[Jonathan Cameron via <qemu-devel@nongnu.org>]

On Mon, 10 Oct 2022 11:20:05 -0400
Gregory Price <gregory.price@memverge.com> wrote:

> > > 
> > > Maybe we should consider 2 new options:
> > > --persistent-memdevs=pm1 pm2 pm3
> > > --volatile-memdevs=vm1 vm2 vm3
> > > 
> > > etc, and deprecate --memdev, and go with your array of memdevs idea.
> > > 
> > > I think I could probably whip that up in a day or two.  Thoughts?  
> > 
> > I wonder if we care to emulate beyond 1 volatile and 1 persistent.
> > Sure devices might exist, but if we can exercise all the code paths
> > with a simpler configuration, perhaps we don't need to handle the
> > more complex ones?
> > 
> > The sticky corner here is Set Partition Info 
> > CXL r3.0 8.2.9.8.2.1
> > 
> > Separation between volatile and non volatile is configurable at runtime.
> >   
> 
> Set Partition Info (Opcode 4101h)
> ... snip ... Paritionable capacity is equal to
> (total capacity - (volatile-only) - (persistent-only))
> 
> By definition, volatile backed memory can only contain volatile regions.
> That's simple.

I guess that's a reasonable model for the QEMU command line.

> 
> For persistent-backed memory, capacity can be chopped up (iif Identify
> Memory Device reports Paritition Alignment to be non-zero).
> 
> --persistent-memdev=pmem-backing
> --volatile-memdev=vmem-backing
> 
> By default, maybe we should make the entire size of persistent regions
> to be persistent-only for the initial prototype. 

Agreed.  Partitioning wasn't high on my todo list though maybe it is
for others...


> This would let us
> merge without support for Set Partition Info.  Then add an additional
> argument to enable additional argument to set the partition alignment.
> 
> --partitionable-pmem=
>   : Defaults to 0. If non-zero, up to N XBytes of PMem may be
>     used as vmem by the operating system at runtime.
> 
> On the CXL tool side we should then see the following output for these
> settings:
> 
> --persistent-memdev=pmem0     1GB
> --volatile-memdev=vmem0       1GB
> --partitionable-pmem=512M
> 
> Capacities:
> ----------------
> Total Capacity: 2GB
> Volatile Capacity: 1GB
> Persistent Capacity: 1GB
> Partitionable Capacity: 512MB
> Partitioned Memory: 0MB
> 
> Available Memory:
> -----------------
> Volatile Memory: 1GB
> Persistent Memory: 1GB
> 
> 
> Then Run something along the lines of:
> `cxl partition-pmem 512MB`   (or whatever args are needed)
> 
> 
> Capacities:
> ----------------
> Total Capacity: 2GB
> Volatile Capacity: 1GB
> Persistent Capacity: 1GB
> Partitionable Capacity: 512MB
> Partitioned Memory: 512MB
> 
> Available Memory:
> -----------------
> Volatile Memory: 1.5GB
> Persistent Memory: 512MB
> 
> 
> Thoughts?

Looks good to me.  Alison, I think you were looking at the partitioning stuff...
What do you think of this?

Jonathan