Re: [Xen-devel] [PATCH for-4.6] p2m/ept: Set the A bit only if PML is enabled

[Tim Deegan <tim@xxxxxxx>]

At 16:18 +0100 on 23 Sep (1443025126), Wei Liu wrote:
> With the discussion still not finalised I'm a bit worried that this
> issue will block the release.
> 
> I think we have a few options here. I will list them in order of my
> preference. Please correct me if I'm talking non-sense, and feel free to
> add more options if I miss anything.
> 
> 1. Disable PML on broken chips, gate access to A bit (or AD) with PML.

I don't much like tying this to PML: this is not a PML-related bug and
there may be CPUs that have A/D but not PML.

Better to have a global read-mostly bool cpu_has_vmx_ept_broken_access_bit,
or whatever name the maintainers prefer. :)

> 2. Implement general framework to detect broken chips and apply quirks.
> 
> I take that there is no general framework at the moment, otherwise the
> patch would have used that.

We already have code that detects specific chips and adjusts things,
e.g. init_intel() in arch/x86/cpu/intel.c.  That seems like a good
place to set the global flag above, or...

> I think Tim's suggestion fall into this category.

...to set a flag disabling EPT.  I suspect people will prefer the
former; it depends on how many of these chips there are out there.

Tim.

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