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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 21.07.15 at 09:05, <kevin.tian@xxxxxxxxx> wrote:
>>  From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
>> Sent: Tuesday, July 21, 2015 2:57 PM
>> >>> On 21.07.15 at 02:57, <kevin.tian@xxxxxxxxx> wrote:
>> >>  From: Andrew Cooper [mailto:amc96@xxxxxxxxxxxxxxxx] 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.

Jan


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