Re: [RFC Qemu PATCH v2 1/2] spapr: drc: Add support for async hcalls at the drc level

[David Gibson]

On Tue, Jan 19, 2021 at 12:40:31PM +0530, Shivaprasad G Bhat wrote:
> Thanks for the comments!
> 
> 
> On 12/28/20 2:08 PM, David Gibson wrote:
> 
> > On Mon, Dec 21, 2020 at 01:08:53PM +0100, Greg Kurz wrote:
> ...
> > > The overall idea looks good but I think you should consider using
> > > a thread pool to implement it. See below.
> > I am not convinced, however.  Specifically, attaching this to the DRC
> > doesn't make sense to me.  We're adding exactly one DRC related async
> > hcall, and I can't really see much call for another one.  We could
> > have other async hcalls - indeed we already have one for HPT resizing
> > - but attaching this to DRCs doesn't help for those.
> 
> The semantics of the hcall made me think, if this is going to be
> re-usable for future if implemented at DRC level.

It would only be re-usable for operations that are actually connected
to DRCs.  It doesn't seem to me particularly likely that we'll ever
have more asynchronous hcalls that are also associated with DRCs.

> Other option
> is to move the async-hcall-state/list into the NVDIMMState structure
> in include/hw/mem/nvdimm.h and handle it with machine->nvdimms_state
> at a global level.

I'm ok with either of two options:

A) Implement this ad-hoc for this specific case, making whatever
simplifications you can based on this specific case.

B) Implement a general mechanism for async hcalls that is *not* tied
to DRCs.  Then use that for the existing H_RESIZE_HPT_PREPARE call as
well as this new one.

> Hope you are okay with using the pool based approach that Greg

Honestly a thread pool seems like it might be overkill for this
application.

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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