Re: [PATCH v6 15/20] vfio/mdev: idxd: ims domain setup for the vdcm

[Dave Jiang]

On 5/26/2021 5:54 PM, Jason Gunthorpe wrote:
> On Wed, May 26, 2021 at 05:22:22PM -0700, Dave Jiang wrote:
> > On 5/23/2021 4:50 PM, Jason Gunthorpe wrote:
> > > On Fri, May 21, 2021 at 05:20:37PM -0700, Dave Jiang wrote:
> > > > @@ -77,8 +80,18 @@ int idxd_mdev_host_init(struct idxd_device *idxd, struct mdev_driver *drv)
> > > >    		return rc;
> > > >    	}
> > > > +	ims_info.max_slots = idxd->ims_size;
> > > > +	ims_info.slots = idxd->reg_base + idxd->ims_offset;
> > > > +	idxd->ims_domain = pci_ims_array_create_msi_irq_domain(idxd->pdev, &ims_info);
> > > > +	if (!idxd->ims_domain) {
> > > > +		dev_warn(dev, "Fail to acquire IMS domain\n");
> > > > +		iommu_dev_disable_feature(dev, IOMMU_DEV_FEAT_AUX);
> > > > +		return -ENODEV;
> > > > +	}
> > > I'm quite surprised that every mdev doesn't create its own ims_domain
> > > in its probe function.
> > >
> > > This places a global total limit on the # of vectors which makes me
> > > ask what was the point of using IMS in the first place ?
> > >
> > > The entire idea for IMS was to make the whole allocation system fully
> > > dynamic based on demand.
> > Hi Jason, thank you for the review of the series.
> >
> > My understanding is that the driver creates a single IMS domain for the
> > device and provides the address base and IMS numbers for the domain based on
> > device IMS resources. So the IMS region needs to be contiguous. Each mdev
> > can call msi_domain_alloc_irqs() and acquire the number of IMS vectors it
> > desires and the DEV MSI core code will keep track of which vectors are being
> > used. This allows the mdev devices to dynamically allocate based on demand.
> > If the driver allocates a domain per mdev, it'll needs to do internal
> > accounting of the base and vector numbers for each of those domains that the
> > MSI core already provides. Isn't that what we are trying to avoid? As mdevs
> > come and go, that partitioning will become fragmented.
> I suppose it depends entirely on how the HW works.
>
> If the HW has a fixed number of interrupt vectors organized in a
> single table then by all means allocate a single domain that spans the
> entire fixed HW vector space. But then why do we have a ims_size
> variable here??
>
> However, that really begs the question of why the HW is using IMS at
> all? I'd expect needing 2x-10x the max MSI-X vector size before
> reaching for IMS.
>
> So does IDXD really have like a 4k - 40k entry linear IMS vector table
> to wrap a shared domain around?
>
> Basically, that isn't really "scalable" it is just "bigger".
>
> Fully scalable would be for every mdev to point to its own 2k entry
> IMS table that is allocated on the fly. Every mdev gets a domain and
> every domain is fully utilized by the mdev in emulating
> MSI-X. Basically for a device like idxd every PASID would have to map
> to a IMS vector table array.
>
> I suppose that was not what was done?

At least not for first gen of hardware. DSA 1.0 has 2k of IMS entries 
total. ims_size is what is read from the device cap register. For MSIX, 
the device only has 1 misc vector and 8 I/O vectors. That's why IMS is 
being used for mdevs. We will discuss with our hardware people your 
suggestion.

> Jason