Re: [PATCH] hw/vfio/common: Trace in which mode a IOMMU is opened

[Peter Xu]

On Wed, May 27, 2020 at 06:27:38PM +0200, Philippe Mathieu-Daudé wrote:
> On 5/27/20 6:16 PM, Peter Xu wrote:
> > On Wed, May 27, 2020 at 05:53:16PM +0200, Philippe Mathieu-Daudé wrote:
> >>>>> +    for (i = 0; i < ARRAY_SIZE(iommu); i++) {
> >>>>> +        if (ioctl(container->fd, VFIO_CHECK_EXTENSION, iommu[i].type)) 
> >>>>> {
> >>>>> +            trace_vfio_get_iommu_type(iommu[i].type, iommu[i].name);
> >>>> Just wondering why you want to trace the type as you now have the name
> >>>> string.
> >>>
> >>> You are right :)
> >>>
> >>>>> +            return iommu[i].type;
> >>>>>          }
> >>>>>      }
> >>>>> +    trace_vfio_get_iommu_type(-1, "Not available or not supported");
> >>>> nit: from a debugging pov, this may be not needed as
> >>>> vfio_get_group/vfio_connect_container() fails and this leads to an error
> >>>> output.
> >>
> >> But you can reach this for example using No-IOMMU. If you don't mind, I
> >> find having this information in the trace log clearer.
> > 
> > I kinda agree with Eric - AFAICT QEMU vfio-pci don't work with no-iommu, 
> > then
> > it seems meaningless to trace it...
> > 
> > I'm not sure whether this trace is extremely helpful because syscalls like 
> > this
> > could be easily traced by things like strace or bpftrace as general tools 
> > (and
> > this information should be a one-time thing rather than dynamically 
> > changing),
> > no strong opinion though.  Also, if we want to dump something, maybe it's
> > better to do in vfio_init_container() after vfio_get_iommu_type() 
> > succeeded, so
> > we dump which container is enabled with what type of iommu.
> 
> OK. I'm a recent VFIO user so maybe I am not using the good information.
> 
> This trace helps me while working on a new device feature, I didn't
> thought about gathering it in a production because there I'd expect
> things to work.
> 
> Now in my case what I want is to know is if I'm using a v1 or v2 type.
> Maybe this information is already available in /proc or /sys and we
> don't need this patch...

I don't know such /proc or /sys, so maybe it's still useful. I guess Alex would
have the best judgement. The strace/bpftrace things are not really reasons I
found to nak this patch, but just something I thought first that could be
easier when any of us wants to peak at those information, probably something
just FYI. :-)

Thanks,

-- 
Peter Xu