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

[Ian Jackson <iwj@xxxxxxxxxxxxxx>]

Jan Beulich writes ("Re: [PATCH][4.15] x86/shadow: suppress "fast fault path" 
optimization without reserved bits"):
> On 26.02.2021 18:07, Tim Deegan wrote:
> > Yes, I think it could be reduced to use just one reserved address bit.
> > IIRC we just used such a large mask so the magic entries would be
> > really obvious in debugging, and there was no need to support arbitrary
> > address widths for emulated devices.
> 
> Will cook a patch, albeit I guess I'll keep as many of the bits set
> as possible, while still being able to encode a full-40-bit GFN.
> 
> Ian - I don't suppose you'd consider this a reasonable thing to do
> for 4.15? It would allow limiting the negative (performance) effect
> the change here has.

I'm afraid I don't follow enough of the background here to have an
opinion right now.  Can someone explain to me the risks (and,
correspondingly, upsides) of the options ?  Sorry to be dim, I don't
seem to be firing on all cylinders today.

Ian.