Re: [PATCH] VT-d: add iommu=igfx_off option to workaround graphics issues

["Tian, Kevin" <kevin.tian@intel.com>]

> From: Jan Beulich [mailto:JBeulich@suse.com]
> Sent: Tuesday, July 21, 2015 3:17 PM
 
> >>> On 21.07.15 at 09:05, <kevin.tian@intel.com> wrote:
> >>  From: Jan Beulich [mailto:JBeulich@suse.com]
> >> Sent: Tuesday, July 21, 2015 2:57 PM
> >> >>> On 21.07.15 at 02:57, <kevin.tian@intel.com> wrote:
> >> >>  From: Andrew Cooper [mailto:amc96@hermes.cam.ac.uk] On Behalf Of Andrew
> >> > Cooper
> >> >> Sent: Monday, July 20, 2015 4:21 PM
> >> > This is the part which I don't quite understand. WC is essentially an UC
> >> > attribute with write buffer to accelerate the write efficiency. There
> >> > should be no correctness problem to use either WC or UC if i915 driver
> >> > wants WC.
> >>
> >> "Should" is too weak a term here: Using WC on the wrong piece of
> >> memory or without the necessary fencing can imo very well cause
> >> correctness problems (which would be hidden by WC -> UC
> >> conversion behind the driver's back).
> >>
> >
> > My point is about when i915 wants WC, then either UC (I suppose is
> > the case before that Linux commit) and WC (by that commit) has
> > no correctness problem. UC is more strict than WC. It's just performance
> > difference. It's not about using WC in wrong place when it's not desired.
> 
> In this you assume there are no misguided attempts to request
> WC in the driver, which would have gone unnoticed as long as WC
> didn't become the effective attribute. And "misguided" here is
> meant to include cases where hardware errata may need taking
> care of.
> 

I don't understand this point. If it's misguided attempts it'd be
same on bare metal. 

Thanks
Kevin