RE: [PATCH v4 3/6] iommu/sva: Stop using ioasid_set for SVA

[Tian, Kevin]

> From: Jacob Pan <jacob.jun.pan@xxxxxxxxxxxxxxx>
> Sent: Wednesday, March 8, 2023 1:42 AM
> 
> Hi Jason,
> 
> On Fri, 3 Mar 2023 09:15:45 -0400, Jason Gunthorpe <jgg@xxxxxxxxxx>
> wrote:
> 
> > On Fri, Mar 03, 2023 at 05:57:41PM +0800, Baolu Lu wrote:
> > > On 2023/3/3 17:32, Jean-Philippe Brucker wrote:
> > > > > I suppose the common thing is reserving some kind of special
> > > > > PASIDs.
> > > > Are you planning to use RID_PASID != 0 in VT-d?  Otherwise we could
> > > > just communicate min_pasid from the IOMMU driver the same way we
> do
> > > > max_pasid.
> > > >
> > > > Otherwise I guess re-introduce a lighter ioasid_alloc() that the IOMMU
> > > > driver calls to reserve PASID0/RID_PASID.
> > >
> > > Yes. We probably will use a non-zero RID_PASID in the future. An
> > > interface to reserve (or allocate) a PASID from iommu_global_pasid_ida
> > > should work then.
> >
> > Just allowing the driver to store XA_ZERO_ENTRY would be fine
> >
> So we provide APIs for both?
> 1. alloc a global PASID, returned by this API
> 2. try to reserve a global PASID given by the driver, i.e.
> 	xa_cmpxchg(&iommu_global_pasid_ida.xa, 2, NULL,
> XA_ZERO_ENTRY,
> 			 GFP_KERNEL);
> seems #1 is sufficient.
> 

No need for both. There will be on-demand allocation on this global
space so a reservation interface doesn't make sense.