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

[Guo Ren]

Hi,

On Mon, Sep 16, 2019 at 8:57 PM Jean-Philippe Brucker
<jean-philippe@xxxxxxxxxx> wrote:
> On 13/09/2019 09:13, Guo Ren wrote:
> > Another idea is seperate remote TLB invalidate into two instructions:
> >
> >  - sfence.vma.b.asyc
> >  - sfence.vma.b.barrier // wait all async TLB invalidate operations
> > finished for all harts.
>
> It's not clear to me how this helps, but I probably don't have the whole
> picture. If you have a place where it is safe to wait for the barrier to
> complete, why not do the whole invalidate there?
>
> > (I remember who mentioned me separate them into two instructions after
> > session. Anup? Is the idea right ?)
Forget it, I still use irq signal in my formal proposal [1]. I also
couldn't image the whole picture :P


> >     To solve the problem, we could define a async mode in sfence.vma.b to
> >     slove the problem and finished with per_cpu_irq/exception.
>
> 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"

IOASID != ASID for CPU_TLB and IOMMU_TLB.

When you say "since the IOASID doesn't change",Is it PASID or my IOASID ? -_*!
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.

>
> 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/