Re: [Qemu-devel] [qemu PATCH v4 3/4] nvdimm, acpi: support NFIT platform capabilities

[Ross Zwisler]

On Wed, Jun 06, 2018 at 11:00:33AM -0600, Ross Zwisler wrote:
> On Wed, Jun 06, 2018 at 10:50:51AM -0600, Ross Zwisler wrote:
> > On Tue, Jun 05, 2018 at 03:21:30PM -0700, Dan Williams wrote:
> > > On Tue, Jun 5, 2018 at 3:07 PM, Ross Zwisler
> > > <address@hidden> wrote:
> > > > On Tue, Jun 05, 2018 at 09:37:25PM +0300, Michael S. Tsirkin wrote:
> > > >> On Tue, Jun 05, 2018 at 11:15:00AM -0700, Dan Williams wrote:
> > > >> > On Tue, Jun 5, 2018 at 9:42 AM, Ross Zwisler
> > > >> > <address@hidden> wrote:
> > > >> > > On Tue, Jun 05, 2018 at 06:25:27PM +0300, Michael S. Tsirkin wrote:
> > > >> > >> On Mon, May 21, 2018 at 10:32:02AM -0600, Ross Zwisler wrote:
> > > >> > >> > Add a machine command line option to allow the user to control 
> > > >> > >> > the Platform
> > > >> > >> > Capabilities Structure in the virtualized NFIT.  This Platform 
> > > >> > >> > Capabilities
> > > >> > >> > Structure was added in ACPI 6.2 Errata A.
> > > >> > >> >
> > > >> > >> > Signed-off-by: Ross Zwisler <address@hidden>
> > > >> > >>
> > > >> > >> I tried playing with it and encoding the capabilities is
> > > >> > >> quite awkward.
> > > >> > >>
> > > >> > >> Can we add bits for specific capabilities instead of nvdimm-cap?
> > > >> > >>
> > > >> > >> How about:
> > > >> > >>
> > > >> > >> "cpu-flush-on-power-loss-cap"
> > > >> > >> "memory-flush-on-power-loss-cap"
> > > >> > >> "byte-addressable-mirroring-cap"
> > > >> > >
> > > >> > > Hmmm...I don't like that as much because:
> > > >> > >
> > > >> > > a) It's very verbose.  Looking at my current qemu command line few 
> > > >> > > other
> > > >> > >    options require that many characters, and you'd commonly be 
> > > >> > > defining more
> > > >> > >    than one of these for a given VM.
> > > >> > >
> > > >> > > b) It means that the QEMU will need to be updated if/when new 
> > > >> > > flags are added,
> > > >> > >    because we'll have to have new options for each flag.  The 
> > > >> > > current
> > > >> > >    implementation is more future-proof because you can specify any 
> > > >> > > flags
> > > >> > >    value you want.
> > > >> > >
> > > >> > > However, if you feel strongly about this, I'll make the change.
> > > >> >
> > > >> > Straw-man: Could we do something similar with what we are doing in 
> > > >> > ndctl?
> > > >> >
> > > >> > enum ndctl_persistence_domain {
> > > >> >         PERSISTENCE_NONE = 0,
> > > >> >         PERSISTENCE_MEM_CTRL = 10,
> > > >> >         PERSISTENCE_CPU_CACHE = 20,
> > > >> >         PERSISTENCE_UNKNOWN = INT_MAX,
> > > >> > };
> > > >> >
> > > >> > ...and have the command line take a number where "10" and "20" are
> > > >> > supported today, but allows us to adapt to new persistence domains in
> > > >> > the future.
> > > >>
> > > >> I'm fine with that except can we have symbolic names instead of numbers
> > > >> on command line?
> > > >>
> > > >> --
> > > >> MST
> > > >
> > > > Okay, we can move to the symbolic names.  Do you want them to be that 
> > > > long, or
> > > > would:
> > > >
> > > > nvdimm-cap-cpu
> > > > nvdimm-cap-mem-ctrl
> > > > nvdimm-cap-mirroring
> > > 
> > > Wait, why is mirroring part of this?
> > > 
> > > I was thinking this option would be:
> > > 
> > >     --persistence-domain={cpu,mem-ctrl}
> > > 
> > > ...and try not to let ACPI specifics leak into the qemu command line
> > > interface. For example PowerPC qemu could have a persistence domain
> > > communicated via Open Firmware or some other mechanism.
> > 
> > Sure, this seems fine, though we may want to throw an "nvdimm" in the name
> > somewhere so it's clear what the option affects.
> > 
> > nvdimm-persistence={cpu,mem-ctrl} maybe?
> > 
> > Michael, does this work for you?
> 
> Hmm...also, the version of these patches that used numeric values did go
> upstream in QEMU.  So, do we need to leave that interface intact, and just add
> a new one that uses symbolic names?  Or did you still just want to replace the
> numeric one since it hasn't appeared in a QEMU release yet?

I guess another alternative would be to just add symbolic name options to the
existing command line option, i.e. allow all of these:

nvdimm-cap=3            # CPU cache
nvdimm-cap=cpu          # CPU cache
nvdimm-cap=2            # memory controller
nvdimm-cap=mem-ctrl     # memory controller

And just have them as aliases.  This retains backwards compatibility with
what is already there, allows for other numeric values without QEMU updates if
other bits are defined (though we are still tightly tied to ACPI in this
case), and provides a symbolic name which is easier to use.

Thoughts?