Re: [Qemu-devel] [qemu-s390x] [PATCH 1/2] vfio-ccw: add force unlimited prefetch property

[Cornelia Huck <cohuck@redhat.com>]

On Wed, 16 May 2018 18:42:01 +0200
Halil Pasic <pasic@linux.ibm.com> wrote:

> On 05/14/2018 06:04 PM, Cornelia Huck wrote:
> > On Mon, 14 May 2018 16:22:30 +0200
> > Halil Pasic <pasic@linux.ibm.com> wrote:
> >   
> >> On 05/14/2018 03:45 PM, Cornelia Huck wrote:  
> >>> On Mon, 14 May 2018 14:40:13 +0200
> >>> Halil Pasic <pasic@linux.ibm.com> wrote:
> >>>      
> >>>> On 05/14/2018 02:18 PM, Cornelia Huck wrote:  
> >>>>> On Thu, 10 May 2018 02:07:11 +0200
> >>>>> Halil Pasic <pasic@linux.ibm.com> wrote:  
> 
> [..]
> 
> >>>>>> +    bool f_upfch;  
> >>>>>
> >>>>> force_unlimited_prefetch? You only use it that often :)
> >>>>>         
> >>>>
> >>>> I would have expected complaints for the property name in the
> >>>> first place. I think we should first find a good name for the
> >>>> property and then consider the rest.  
> >>>
> >>> What about 'force_pfch' (at least matches the name of the bit in the
> >>> code)?
> >>>      
> >>
> >> I like upfch more as it really not about forcing any prefetch, but
> >> allowing *unlimited* prefetch for the channel program.  
> > 
> > 'always_allow_prefetch', then? The problem is that we force a flag to
> > be set, which does not force but allow something. Hard to express in a
> > short property name :(
> > 
> > Any other suggestions?
> >   
> 
> How about force-orb-pfch or force-orb-pbit (PoP name) for the property
> and with underscores for the variable. My idea was (starting from your
> force_pfch) to spell out that we are fiddling with that orb bit.

force-orb-pfch looks reasonable to me.

> 
> Can I/do I have to document the property somewhere? Telling the whole
> story with the name is going to be difficult.

The description member would require that you define your own property
type IIUC, which I think would be overkill. So I'd add a comment in the
source code.

> 
> >>  
> >>>>     
> >>>>>>     } VFIOCCWDevice;  
> >   
> >>>>>> @@ -429,6 +441,7 @@ static void vfio_ccw_unrealize(DeviceState *dev, Error **errp)
> >>>>>>     
> >>>>>>     static Property vfio_ccw_properties[] = {
> >>>>>>         DEFINE_PROP_STRING("sysfsdev", VFIOCCWDevice, vdev.sysfsdev),
> >>>>>> +    DEFINE_PROP_BOOL("f-upfch", VFIOCCWDevice, f_upfch, false),  
> >>>>>  
> 
> [..]
> 
> >>>
> >>> Another thought: Should there be a warning logged somewhere if we
> >>> actually force pfch (i.e., not just set the property)?
> >>>      
> >>
> >> I don't think so. With libvirt the cmd line gets logged. So we can
> >> tell if the machine was running with this forced or not. This knob
> >> is really (an opt-in) for expert users only.  
> > 
> > But there's a difference between 'we set this one preemptively' and 'we
> > set it, and the guest actually did a request with pfch off'.
> >   
> 
> I expect the admin to set it *only* after seeing SSCHs fail with
> the 'vfio-ccw requires PFCH flag set' message. So no preemptive usage
> is expected, but...
> 
> >>
> >> Furthermore a warning about this may not be very constructive,
> >> as there is not much that can be done to make the warning go away.
> >> IMHO getting used to warnings is not a good thing.
> >>
> >> Or am I missing a reason for issuing a warning?  
> > 
> > Just log this once so that the admin sees 'yes, the guest actually did
> > a request with pfch off, so if funny things happened, that might be the
> > reason'. Of course, if this is only an edge use case, that would be
> > overkill.
> >   
> 
> ... if the admin (actually more likely developer/tester) is mistaken
> about the guest, and it does require the guarantees, but things don't blow
> up right away, this message, together with the timestamp could help
> determine why things turned funny.
> 
> So I do see the benefit now. But then I wonder if it should be a
> WARN_ONCE type thing, or if we shall issue a warning each time the override
> happens. (Considering the laid out expected benefit, if the first request
> is OK but some subsequent one is not OK (needs the conservative prefetch
> behavior) we don't gain much).

A WARN_ONCE (maybe per device) would be the sanest option, I think.