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

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Date:   Thu, 30 Sep 2021 19:23:55 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>
Cc:     Alex Williamson <alex.williamson@...hat.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>, "hch@....de" <hch@....de>,
        "jasowang@...hat.com" <jasowang@...hat.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "jean-philippe@...aro.org" <jean-philippe@...aro.org>,
        "parav@...lanox.com" <parav@...lanox.com>,
        "lkml@...ux.net" <lkml@...ux.net>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "lushenming@...wei.com" <lushenming@...wei.com>,
        "eric.auger@...hat.com" <eric.auger@...hat.com>,
        "corbet@....net" <corbet@....net>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "yi.l.liu@...ux.intel.com" <yi.l.liu@...ux.intel.com>,
        "Tian, Jun J" <jun.j.tian@...el.com>, "Wu, Hao" <hao.wu@...el.com>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        "jacob.jun.pan@...ux.intel.com" <jacob.jun.pan@...ux.intel.com>,
        "kwankhede@...dia.com" <kwankhede@...dia.com>,
        "robin.murphy@....com" <robin.murphy@....com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "dwmw2@...radead.org" <dwmw2@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "baolu.lu@...ux.intel.com" <baolu.lu@...ux.intel.com>,
        "david@...son.dropbear.id.au" <david@...son.dropbear.id.au>,
        "nicolinc@...dia.com" <nicolinc@...dia.com>
Subject: Re: [RFC 10/20] iommu/iommufd: Add IOMMU_DEVICE_GET_INFO

On Thu, Sep 30, 2021 at 09:35:45AM +0000, Tian, Kevin wrote:

> > The Intel functional issue is that Intel blocks the cache maintaince
> > ops from the VM and the VM has no way to self-discover that the cache
> > maintaince ops don't work.
> 
> the VM doesn't need to know whether the maintenance ops 
> actually works.

Which is the whole problem.

Intel has a design where the device driver tells the device to issue
non-cachable TLPs.

The driver is supposed to know if it can issue the cache maintaince
instructions - if it can then it should ask the device to issue
no-snoop TLPs.

For instance the same PCI driver on non-x86 should never ask the
device to issue no-snoop TLPs because it has no idea how to restore
cache coherence on eg ARM.

Do you see the issue? This configuration where the hypervisor silently
make wbsync a NOP breaks the x86 architecture because the guest has no
idea it can no longer use no-snoop features.

Using the IOMMU to forcibly prevent the device from issuing no-snoop
makes this whole issue of the broken wbsync moot.

It is important to be really clear on what this is about - this is not
some idealized nice iommu feature - it is working around alot of
backwards compatability baggage that is probably completely unique to
x86.

> > Other arches don't seem to have this specific problem...
> 
> I think the key is whether other archs allow driver to decide DMA
> coherency and indirectly the underlying I/O page table format. 
> If yes, then I don't see a reason why such decision should not be 
> given to userspace for passthrough case.

The choice all comes down to if the other arches have cache
maintenance instructions in the VM that *don't work*

Jason