Re: [PATCH 1/2][4.15?] x86/shadow: suppress "fast fault path" optimization when running virtualized

[Tim Deegan <tim@xxxxxxx>]

At 16:37 +0100 on 05 Mar (1614962224), Jan Beulich wrote:
> We can't make correctness of our own behavior dependent upon a
> hypervisor underneath us correctly telling us the true physical address
> with hardware uses. Without knowing this, we can't be certain reserved
> bit faults can actually be observed. Therefore, besides evaluating the
> number of address bits when deciding whether to use the optimization,
> also check whether we're running virtualized ourselves.
> 
> Requested-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

Acked-by: Tim Deegan <tim@xxxxxxx>

I would consider this to be a bug in the underlying hypervisor, but I
agree than in practice it won't be safe to rely on it being correct.

These checks are getting fiddly now.  I think that if we end up adding
any more to them it might be good to set a read-mostly variable at boot
time rather than do them on every MMIO/NP fault.

Cheers,

Tim.