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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 08.03.2021 10:25, Tim Deegan wrote:
> 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>

Thanks.

> 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.

Suffice it to say that I don't think we present a correct value to
our guests. Plus, as said elsewhere, what would you suggest to hand
to the guest in case it may need migrating (to a host with a
different number of PA bits)?

> 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.

Maybe, but I'd like to point out that the fault path uses only the
sh_l1e_is_*() functions (plus sh_l1e_mmio_get_gfn()), and hence
isn't affected by the added fiddly-ness.

Jan