Re: [Intel-gfx] [PATCH v3 12/12] vfio/pci: Report dev_id in VFIO_DEVICE_GET_PCI_HOT_RESET_INFO

From: Alex Williamson <alex.williamson@redhat.com>
To: "Liu, Yi L" <yi.l.liu@intel.com>
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Subject: Re: [Intel-gfx] [PATCH v3 12/12] vfio/pci: Report dev_id in VFIO_DEVICE_GET_PCI_HOT_RESET_INFO
Date: Wed, 26 Apr 2023 07:20:20 -0600	[thread overview]
Message-ID: <20230426072020.62834ac6.alex.williamson@redhat.com> (raw)
In-Reply-To: <DS0PR11MB75298CDC8108BA213243DBB8C3659@DS0PR11MB7529.namprd11.prod.outlook.com>

On Wed, 26 Apr 2023 07:22:17 +0000
"Liu, Yi L" <yi.l.liu@intel.com> wrote:

> > From: Alex Williamson <alex.williamson@redhat.com>
> > Sent: Thursday, April 20, 2023 10:09 PM  
> [...]
> > > > Whereas dev-id < 0
> > > > (== -1) is an affected device which prevents hot-reset, ex. an un-owned
> > > > device, device configured within a different iommufd_ctx, or device
> > > > opened outside of the vfio cdev API."  Is that about right?  Thanks,  
> > >
> > > Do you mean to have separate err-code for the three possibilities? As
> > > the devid is generated by iommufd and it is u32. I'm not sure if we can
> > > have such err-code definition without reserving some ids in iommufd.  
> > 
> > Yes, if we're going to report the full dev-set, I think we need at
> > least two unique error codes or else the user has no way to determine
> > the subset of invalid dev-ids which block the reset.  I think Jason is
> > proposing the set of valid dev-ids are >0, a dev-id of zero indicates
> > some form of non-blocking, while <0 (or maybe specifically -1)
> > indicates a blocking device.  I was trying to get consensus on a formal
> > definition of each of those error codes in my previous reply.  Thanks,  
> 
> Seems like RESETTABLE flag is not needed if we report -1 for the devices
> that block hotreset. Userspace can deduce if the calling device is resettable
> or not by checking if there is any -1 in the affected device list.

There is some redundancy there, yes.  Given the desire for a null array
on the actual reset ioctl I assumed there would also be a desire to
streamline the info ioctl such that userspace isn't required to parse
the return array, for example maybe userspace isn't required to pass a
full buffer and can get the reset availability status from only the
header.  Of course it's still the responsibility of userspace to know
the extent of the reset.  Thanks,

Alex