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

[Dan Williams]

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.