Re: [RFC] Introduce dynamic PPAT allocation for GVT-g

Re: [RFC] Introduce dynamic PPAT allocation for GVT-g

[Chris Wilson]

Quoting Zhi Wang (2017-08-14 13:04:34)
> [ BACKGROUND ]
> 
> Due to different guests will have different PPAT configuration under 
> virtualization environment, for example, Windows guest vs Linux guest, a 
> possible idea to handle this open is
> 
> 1)Letting host have the PPAT entries needed by guest

More or less requires static entries forevermore, locking the guest and
host into not changing in future. That doesn't seem a viable assumption.
 
> 2)Mapping PPAT indexes in guest page table to the corresponding host 
> PPAT entries during building the shadow page table.
> 
> To achieve 1), one approach which has been used before is statically 
> adding the collection of guest PPAT configuration into host i915. So 
> when performing 2) GVT-g will always find an available PPAT entry to map.
> 
> Another possible approach is GVT-g could dynamically find/add/delete the 
> PPAT configuration for different guests. To prevent regression of host 
> i915 due to this changes, it’s better to have two types of PPAT entry: 
> one is “static PPAT entry”, which is to keep current host i915 PPAT 
> configuration unchanged, e.g. the mapping between i915 cache level and 
> PPAT index, etc… just keep everything used by host i915 unchanged and a 
> *static* PPAT entry will not be modified.

Seems reasonable, just something like:

int intel_ppat_find(struct drm_i915_private *i915, struct ppat_config *opts);

That can return the index of an existing ppat entry or assign new one
(or error code, or a struct + ERR_PTR). And with a use-counter
(intel_ppat_get/intel_ppat_put, where intel_ppat_find always returns with
the use-counter preincremented) allows for reconfiguration.

The downside is the remapping required, which I expect will not be too
tricky? As you copy the PTE from the guest into the host you just run it
through a translation table, right?
-Chris
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[RFC] Introduce dynamic PPAT allocation for GVT-g

[Zhi Wang]

[ BACKGROUND ]

Due to different guests will have different PPAT configuration under 
virtualization environment, for example, Windows guest vs Linux guest, a 
possible idea to handle this open is

1)Letting host have the PPAT entries needed by guest

2)Mapping PPAT indexes in guest page table to the corresponding host 
PPAT entries during building the shadow page table.

To achieve 1), one approach which has been used before is statically 
adding the collection of guest PPAT configuration into host i915. So 
when performing 2) GVT-g will always find an available PPAT entry to map.

Another possible approach is GVT-g could dynamically find/add/delete the 
PPAT configuration for different guests. To prevent regression of host 
i915 due to this changes, it’s better to have two types of PPAT entry: 
one is “static PPAT entry”, which is to keep current host i915 PPAT 
configuration unchanged, e.g. the mapping between i915 cache level and 
PPAT index, etc… just keep everything used by host i915 unchanged and a 
*static* PPAT entry will not be modified.

Another type is “dynamic PPAT entry”, which will be managed by GVT-g.

[ EXPECTED CHANGE OF HOST i915 ]

Since i915 doesn’t need dynamic PPAT allocation, one idea is putting all 
the dynamic PPAT entry management in GVT-g, i915 shows a bitmap of 
available PPAT indexes to GVT-g after initializing host PPAT entries, so 
GVT-g could manage them on-demand.

Thanks,
Zhi.
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