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    SubjectRe: [PATCH] intel-iommu: Default to non-coherent for domains unattached to iommus
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    On Fri, 2011-11-11 at 15:49 -0700, Alex Williamson wrote:
    > To fix this, switch domain_update_iommu_coherency() to use the
    > safer, non-coherent default for domains not attached to iommus.

    That isn't a fix for the problem you described.

    The problem is that changing a domain from coherent to non-coherent is
    *broken*. It probably needs to flush the cache for the *entire* set of
    page tables — not just the new context entry it adds.

    You might have removed the *common* case where we trigger that bug, but
    it certainly isn't a fix.

    However, I'd be receptive to an argument that the situation you describe
    is in fact the *only* time we'd have to switch from coherent to
    non-coherent at run time, because the coherency is an all-or-nothing
    characteristic of the chipset. Either all the IOMMUs are coherent, or
    none of them, right? This brain-damage only affects the first chipsets
    from before we worked out that cache incoherency was a *really* f*cking
    stupid idea, doesn't it?

    So if you were to ditch the whole idea of a per-domain runtime update,
    and instead calculate a global value for 'iommu_coherency' at boot time,
    by iterating over for_each_active_iommu()¹, I think that would be a
    better way to deal with the issue. And you *could* really call that a
    'fix'.

    Make sense?

    --
    dwmw2

    ¹ just in *case* they ever differ
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