lkml.org 
[lkml]   [2011]   [Nov]   [11]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [PATCH] intel-iommu: Default to non-coherent for domains unattached to iommus
From
Date
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
[unhandled content-type:application/x-pkcs7-signature]
\
 
 \ /
  Last update: 2011-11-12 01:41    [W:0.098 / U:0.440 seconds]
©2003-2014 Jasper Spaans. Advertise on this site