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

[Shivaprasad G Bhat]

Hi David,

Sorry about the delay.

On 2/8/21 11:51 AM, David Gibson wrote:
> 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.
Okay

> > 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.

I am simplifying it to nvdimm use-case alone and limiting the scope.


> 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.

I think its appropriate here as that is what is being done by virtio-pmem

too for flush requests. The aio infrastructure simplifies lot of the

thread handling usage. Please suggest if you think there are better ways.


I am sending the next version addressing all the comments from you and Greg.


Thanks,

Shivaprasad