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    SubjectRe: [PATCH] intel-iommu: Default to non-coherent for domains unattached to iommus
    * David Woodhouse (dwmw2@infradead.org) wrote:
    > 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.

    For a guest domain, the page tables aren't actually changing.
    And for the snoop mode change, we remap the pages.

    > 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?

    Dunno if it exists going forward (I've stopped being surprised by the
    brain damage in this area ;), but those machines are still out there.

    > 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?

    Ideally, yes. Not sure we can practically do it though. Would have to
    be sure we force incoherent access mode for the busted hw.

    thanks,
    -chris
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