Re: [qemu-s390x] [Qemu-devel] [PATCH RFC] vfio-ap: flag as compatible with balloon

[Halil Pasic]

On Fri, 7 Dec 2018 14:17:20 +0100
Cornelia Huck <address@hidden> wrote:

> On Fri, 7 Dec 2018 13:52:53 +0100
> Halil Pasic <address@hidden> wrote:
> 
> > On Fri, 7 Dec 2018 13:29:46 +0100
> > Cornelia Huck <address@hidden> wrote:
> > 
> > > On Fri, 7 Dec 2018 13:17:02 +0100
> > > Christian Borntraeger <address@hidden> wrote:
> > >   
> > > > On 05.12.2018 15:51, Cornelia Huck wrote:  
> > > > > vfio-ap devices do not pin any pages in the host. Therefore, they
> > > > > are belived to be compatible with memory ballooning.
> > > > > 
> > > > > Flag them as compatible, so both vfio-ap and a balloon can be
> > > > > used simultaneously.
> > > > > 
> > > > > Signed-off-by: Cornelia Huck <address@hidden>    
> > 
> > With the comment stuff sorted out:
> > Reviewed-by: Halil Pasic <address@hidden> 
> 
> So, do you agree with the comment change I suggested?
> 
> +    /*
> +     * vfio-ap devices operate in a way compatible with
> +     * memory ballooning, as no pages are pinned in the host.
> +     * This needs to be set before vfio_get_device() for vfio common to
> +     * handle the balloon inhibitor.
> +     */

I did some digging because my understanding of the problem was
completely insufficient -- now it is just plain insufficient. If I
understood it correctly the crux here seems to be that under certain
circumstances (IOMMU type, presence/absence of domain) vfio locks on
VFIO_IOMMU_MAP, and that vfio_get_group() basically maps it's address
space argument. But for s390x this pinning does not happen.

I mean vfio-ccw does pin pages in the host and is still safe. BTW do
we want to change the message for vfio-cccw?

I intend do some more digging and should I come to some remotely
satisfactory result, I intend to post a short write-up of my
findings here.

I agree to this patch with that commit message despite not having the
clarity, because not having it seems way worse than having it.


> 
> > 
> > @Connie: Just had a look at the MAINTAINERS file and hw/vfio/ap.c
> > is listed under Arch. support S90 with you as a maintainer, and under
> > vfio-ap with 4 maintainers listed one of them being me. The question
> > is who is going to post a PULL request for this?
> 
> General practice has been that I'm collecting everything s390x related.
> I have also pulled from others before (e.g. some bios changes from
> Thomas). While you could apply the patch, send it to me, and then I'd
> queue it to s390-next, I can also simply queue it directly with your
> ack :)
> 
> [Longer term, if you want to collect ap patches and then send me a pull
> request, I would also be happy to do that. For this single patch, it
> seems overkill.]
> 

I agree, it would be an overkill. I guess r-b qualifies as ack.

Regards,
Halil

> > 
> > > > 
> > > > Does it make sense to add cc stable for 3.1?  
> > > 
> > > Can do that, given that s390x systems really rely on the ballooner in
> > > general.
> > >   
> > 
> > I agree with cc stable.
> 
> Will add when applying.
>