On Fri, 2011-11-11 at 16:58 -0800, Chris Wright wrote: > * David Woodhouse (dwmw2@xxxxxxxxxxxxx) wrote: > > On Fri, 2011-11-11 at 16:45 -0800, Chris Wright wrote: > > > > 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. > > > > Why's it not practical? You do the *same* loop we currently have in > > domain_update_iommu_coherency(), except that you do it just *once*, over > > all active IOMMUs, at boot time. And then you just use that result > > forever more. > > Yeah, you're right, that should be simple enough. What about snoop > control? Same thing...should we expect it to be system wide? Because > that one's exported out to KVM and used. Not sure. That might differ for the graphics IOMMU (large page support certainly does). But I don't *think* the coherency does, does it? > > So if *any* IOMMU in the system is non-coherent, you run them all that > > way. > > Minus the measureable slowdown for some devices that were behind coherent > IOMMU (IIRC, the chipsets that had this issue were mobile anyway, so > not as performance sensitive), *nod* . Are there really machines where only *some* of the IOMMUs are coherent? I didn't think there were, which would mean there's no real performance penalty on the hardware that actually exists today. -- dwmw2
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