Re: [PATCH 3/3] x86/ept: force WB cache attributes for grant and foreign maps

[Jan Beulich <jbeulich@xxxxxxxx>]

On 17.06.2021 13:40, Roger Pau Monné wrote:
> On Thu, Jun 17, 2021 at 09:31:28AM +0000, Tian, Kevin wrote:
>>> From: Roger Pau Monne <roger.pau@xxxxxxxxxx>
>>> Sent: Saturday, May 29, 2021 1:40 AM
>>>
>>> Force WB type for grants and foreign pages. Those are usually mapped
>>> over unpopulated physical ranges in the p2m, and those ranges would
>>> usually be UC in the MTRR state, which is unlikely to be the correct
>>> cache attribute. It's also cumbersome (or even impossible) for the
>>> guest to be setting the MTRR type for all those mappings as WB, as
>>> MTRR ranges are finite.
>>>
>>> Note that on AMD we cannot force a cache attribute because of the lack
>>> of ignore PAT equivalent, so the behavior here slightly diverges
>>> between AMD and Intel (or EPT vs NPT/shadow).
>>>
>>> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>>
>> Reviewed-by: Kevin Tian <kevin.tian@xxxxxxxxx>
>>
>> btw incorrect cache attribute brings functional/performance problem. 
>> it'd be good to explain a bit why this problem doesn't hurt AMD in the 
>> commit msg...
> 
> What about re-writing the last commit paragraph as:
> 
> Note that this is not an issue on AMD because WB cache attribute is
> already set on grants and foreign mappings in the p2m and MTRR types
> are ignored. Also on AMD Xen cannot force a cache attribute because of
> the lack of ignore PAT equivalent, so the behavior here slightly
> diverges between AMD and Intel (or EPT vs NPT/shadow).

I'll try to remember to swap this in when committing.

Jan