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

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

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...