linux-kernel - Re: [RFC 10/20] iommu/iommufd: Add IOMMU_DEVICE_GET

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Date:   Fri, 1 Oct 2021 05:28:16 +0200
From:   "hch@....de" <hch@....de>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Jean-Philippe Brucker <jean-philippe@...aro.org>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "jasowang@...hat.com" <jasowang@...hat.com>,
        "kwankhede@...dia.com" <kwankhede@...dia.com>,
        "hch@....de" <hch@....de>, "Jiang, Dave" <dave.jiang@...el.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "corbet@....net" <corbet@....net>,
        "parav@...lanox.com" <parav@...lanox.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        "lkml@...ux.net" <lkml@...ux.net>,
        "david@...son.dropbear.id.au" <david@...son.dropbear.id.au>,
        "dwmw2@...radead.org" <dwmw2@...radead.org>,
        "Tian, Jun J" <jun.j.tian@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "lushenming@...wei.com" <lushenming@...wei.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "robin.murphy@....com" <robin.murphy@....com>
Subject: Re: [RFC 10/20] iommu/iommufd: Add IOMMU_DEVICE_GET_INFO

On Thu, Sep 30, 2021 at 07:04:46PM -0300, Jason Gunthorpe wrote:
> > 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.

Well, it is a combination of the device, the root port and the driver
which all need to be in line to use this.

> 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..

Yes, as usual the GPU folks come up with nasty hacks instead of
providing generic helper.  Basically all we'd need to support it
in a generic way is:

 - a DMA_ATTR_NO_SNOOP (or DMA_ATTR_FORCE_NONCOHERENT to fit the Linux
   terminology) which treats the current dma_map/unmap/sync calls as
   if dev_is_dma_coherent was false
 - a way for the driver to discover that a given architecture / running
   system actually supports this

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

Because no IOMMU in the world can help that fact that a periphal on the
SOC is not part of the cache coherency protocol.