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

Jason Gunthorpe
2021-09-30T22:04:53Z

On Thu, Sep 30, 2021 at 11:33:13AM +0100, Jean-Philippe Brucker wrote:
> 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.

This table is what I was thinking after reading through the ARM docs.

> 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.

The usage of no-snoop is a behavior of a device. A generic PCI driver
should be able to program the device to generate no-snoop TLPs and
ideally rely on an arch specific API in the OS to trigger the required
cache maintenance.

It doesn't make much sense for a portable driver to rely on a
non-portable IO PTE flag to control coherency, since that is not a
standards based approach.

That said, Linux doesn't have a generic DMA API to support
no-snoop. The few GPUs drivers that use this stuff just hardwired
wbsync on Intel..

What I don't really understand is why ARM, with an IOMMU that supports
PTE WB, has devices where dev_is_dma_coherent() == false ?

Is it the case that DMA from those devices ignores the IO PTE's
cachable mode?

Jason