Re: [PATCH] intel-iommu: Default to non-coherent for domains unattached to iommus

[Chris Wright <chrisw@sous-sol.org>]

* Roland Dreier (roland@purestorage.com) wrote:
> On Fri, Nov 11, 2011 at 4:37 PM, David Woodhouse <dwmw2@infradead.org> wrote:
> > 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?
> 
> As we talked about at KS, I have some Westmere EP (ie latest
> and greatest server platform) systems where the BIOS exposes
> an option that allows choosing VT-d coherency on or off, and
> defaults it to "off".

That's just more brain damage AFAICT.  Esp. if you do performance
testing (and choose not to use passthrough mode)... have and it's
quite measurable.  I switched default to on long time ago, w/out
issue.

> What is the "official" Intel line on coherency with Westmere and
> Tylersburg -- because as I also mentioned, I was seeing some
> problems with VT-d and the default "coherency off" setting that
> looked like the IOMMU HW is getting stale PTEs (ie a missing
> or not working cache flush).

That sounds like sw bugs more than official recommendation issue.

thanks,
-chris