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Re: [PATCH][4.15] x86/shadow: suppress "fast fault path" optimization without reserved bits

Jan Beulich writes ("Re: [PATCH][4.15] x86/shadow: suppress "fast fault path" 
optimization without reserved bits"):
> As to 4.15: Without this shadow mode simply won't work on such (new)
> hardware. Hence something needs to be done anyway. An alternative
> would be to limit the change to just the guest-no-present entries (to
> at least allow PV guests to be migrated), and refuse to enable shadow
> mode for HVM guests on such hardware. (In this case we'd probably
> better take care of ...

Thanks for this explanation.

It sounds like the way you have it in this proposed patch is simpler
than the alternative.  And that right now it's not a regression, but
it is needed for running Xen on such newer hardware.

> The main risk here is (in particular for the MMIO part of the change
> I suppose) execution suddenly going a different path, which has been
> unused / untested (for this specific case) for years.

That's somewhat concerning.  But I think this only applies to the new
hardware ?  So it would be risking an XSA but not really risking the
release very much.

I think therefore:

Release-Acked-by: Ian Jackson <iwj@xxxxxxxxxxxxxx>




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