Re: [PATCH RFC 11/14] arm64: Move the ASID allocator code in a separate file

[Jean-Philippe Brucker]

On Thu, Sep 19, 2019 at 09:07:15PM +0800, Guo Ren wrote:
> > The solution I had to this problem is pinning the ASID [1] used by the
> > IOMMU, to prevent the CPU from recycling the ASID on rollover. This way
> > the CPU doesn't have to wait for IOMMU invalidations to complete, when
> > scheduling a task that might not even have anything to do with the IOMMU.
> >
> 
> > In the Arm SMMU, ASID and IOASID (PASID) are separate identifiers. IOASID
> > indexes an entry in the context descriptor table, which contains the ASID.
> > So with unpinned shared ASID you don't need to invalidate the ATC on
> > rollover, since the IOASID doesn't change, but you do need to modify the
> > context descriptor and invalidate cached versions of it.
> The terminology confused me a lot. I perfer use PASID for IOMMU and
> ASID is for CPU.
> Arm's entry of the context descriptor table contains a "IOASID"

The terminology I've been using so far is different:
* IOASID is PASID
* The entry in the context descriptor table contains an ASID, which
  is either "shared" with CPUs or "private" to the SMMU (the SMMU spec
  says "shared" or "non-shared").
* So the CPU and SMMU TLBs use ASIDs, and the PCI ATC uses IOASID

> IOASID != ASID for CPU_TLB and IOMMU_TLB.
> 
> When you say "since the IOASID doesn't change",Is it PASID or my IOASID ? -_*!

I was talking about PASID. Maybe we can drop "IOASID" and talk only
about ASID and PASID :)

> PASID in PCI-sig was used to determine transfer address space.
> For intel, the entry which is indexed by PASID also contain S1/S2.PGD
> and DID(VMID).
> For arm, the entry which is indexed by PASID only contain S1.PGD and
> IOASID. Compare to Intel Vt-d Scalable mode, arm's design can't
> support PCI Virtual Function.

The SMMU does support PCI Virtual Function - an hypervisor can assign a
VF to a guest, and let that guest partition the VF into smaller contexts
by using PASID.  What it can't support is assigning partitions of a PCI
function (VF or PF) to multiple Virtual Machines, since there is a
single S2 PGD per function (in the Stream Table Entry), rather than one
S2 PGD per PASID context.

Thanks,
Jean

> > Once you have pinned ASIDs, you could also declare that IOASID = ASID. I
> > don't remember finding an argument to strictly forbid it, even though ASID
> > and IOASID have different sizes on Arm (respectively 8/16 and 20 bits).
> ASID and IOASID are hard to keep the same between CPU system and IOMMU
> system. So I introduce S1/S2.PGD.PPN as a bridge between CPUs and
> IOMMUs.
> See my proposal [1]
> 
> 1: https://lore.kernel.org/linux-csky/1568896556-28769-1-git-send-email-guoren@xxxxxxxxxx/T/#u
> -- 
> Best Regards
>  Guo Ren
> 
> ML: https://lore.kernel.org/linux-csky/