Re: [RFC 10/20] iommu/iommufd: Add IOMMU_DEVICE_GET_INFO

Jean-Philippe Brucker
2021-09-30T10:33:38Z

On Thu, Sep 30, 2021 at 08:30:42AM +0000, Tian, Kevin wrote:
> > From: Jason Gunthorpe
> > Sent: Wednesday, September 29, 2021 8:37 PM
> >
> > On Wed, Sep 29, 2021 at 08:48:28AM +0000, Tian, Kevin wrote:
> >
> > > ARM:
> > > - set to snoop format if IOMMU_CACHE
> > > - set to nonsnoop format if !IOMMU_CACHE
> > > (in both cases TLP snoop bit is ignored?)
> >
> > Where do you see this? I couldn't even find this functionality in the
> > ARM HW manual??
>
> Honestly speaking I'm getting confused by the complex attribute
> transformation control (default, replace, combine, input, output, etc.)
> in SMMU manual. Above was my impression after last check, but now
> I cannot find necessary info to build the same picture (except below
> code). :/
>
> >
> > What I saw is ARM linking the IOMMU_CACHE to a IO PTE bit that causes
> > the cache coherence to be disabled, which is not ignoring no snoop.
>
> My impression was that snoop is one way of implementing cache
> coherency and now since the PTE can explicitly specify cache coherency
> like below:
>
> else if (prot & IOMMU_CACHE)
> pte |= ARM_LPAE_PTE_MEMATTR_OIWB;
> else
> pte |= ARM_LPAE_PTE_MEMATTR_NC;
>
> This setting in concept overrides the snoop attribute from the device thus
> make it sort of ignored?

To make sure we're talking about the same thing: "the snoop attribute from
the device" is the "No snoop" attribute in the PCI TLP, right?

The PTE flags define whether the memory access is cache-coherent or not.
* WB is cacheable (short for write-back cacheable. Doesn't matter here
what OI or RWA mean.)
* NC is non-cacheable.

| Normal PCI access | No_snoop PCI access
-------+-------------------+-------------------
PTE WB | Cacheable | Non-cacheable
PTE NC | Non-cacheable | Non-cacheable

Cacheable memory access participate in cache coherency. Non-cacheable
accesses go directly to memory, do not cause cache allocation.

On Arm cache coherency is configured through PTE attributes. I don't think
PCI No_snoop should be used because it's not necessarily supported
throughout the system and, as far as I understand, software can't discover
whether it is.

[...]
> Maybe I'll get a clearer picture on this after understanding the difference
> between cache coherency and snoop on ARM.

The architecture uses terms "cacheable" and "coherent". The term "snoop"
is used when referring specifically to the PCI "No snoop" attribute. It is
also used within the interconnect coherency protocols, which are invisible
to software.

Thanks,
Jean