RE: [PATCH 3/4] iommu/sva: Support reservation of global PASIDs

[Tian, Kevin]

> From: Jacob Pan <jacob.jun.pan@xxxxxxxxxxxxxxx>
> Sent: Friday, March 10, 2023 1:06 AM
> 
> Hi Jason,
> 
> On Mon, 6 Mar 2023 15:05:27 -0400, Jason Gunthorpe <jgg@xxxxxxxxxx>
> wrote:
> 
> > On Mon, Mar 06, 2023 at 06:48:43PM +0000, Luck, Tony wrote:
> > > >> ENQCMDS does not have the restriction of using a single CPU MSR to
> > > >> store PASIDs, PASID is supplied to the instruction operand.
> > > >
> > > > Huh? That isn't what it says in the programming manual. It says the
> > > > PASID only comes from the IA32_PASID msr and the only two operands
> are
> > > > the destination MMIO and the memory source for the rest of the
> > > > payload.
> > >
> > > Jason,
> > >
> > > Two different instructions with only one letter different in the name.
> > >
> > > ENQCMD - ring 3 instruction. The PASID is inserted into the descriptor
> > > pushed to the device from the IA32_PASID MSR.
> > >
> > > ENQCMDS - ring 0 instruction (see that trailing "S" for Supervisor
> > > mode). In this case the submitter can include any PASID value they want
> > > in the in-memory copy of the descriptor and ENQCMDS will pass that to
> > > the device.
> >
> > Ah, well, my comment wasn't talking about ENQCMDS :)
> >
> > If ENQCMDS can take in an arbitary PASID then there is no
> > justification here to use the global allocator.
> >
> > The rational is more like:
> >
> >  IDXD uses PASIDs that come from the SVA allocator. It needs to create
> >  an internal kernel-only PASID that is non-overlapping so allow the SVA
> >  allocator to reserve PASIDs for driver use.
> >
> >  IDXD has to use the global SVA PASID allocator beacuse its userspace
> >  will use ENQCMD which requires global PASIDs.
> >
> yes, great summary. I think that is the same as what I was trying to say
> earlier :)
> "due the unforgiving nature of ENQCMD that requires global PASIDs,
> ENQCMDS
> has no choice but to allocate from the same numberspace to avoid conflict."
> 
> In that sense, I feel the global allocator should be staying with SVA
> instead of moving to iommu core (as Kevin suggested). Because we are trying
> to have non-overlapping pasid with SVA.
> 

I still doubt 'reserve' is the right interface to define.

for DMA domain probably yes as it's static and one-off.

but thinking louder when the same driver starts to support SIOV we
need allocating additional PASIDs on demand which is hardly to be
fit in a reservation interface.