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Re: [PATCH 1/2][4.15?] x86/shadow: suppress "fast fault path" optimization when running virtualized
- To: Jan Beulich <jbeulich@xxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Fri, 5 Mar 2021 15:47:00 +0000
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On 05/03/2021 15:37, 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.
I think it would be helpful to point out why we can't even probe at boot
- the behaviour may genuinely change as we migrate, and if we ever end
up on an IceLake system levelled down for compatibility with older CPUs,
the "paddr_bits < PADDR_BITS" check will malfunction in an unsafe way.
>
> Requested-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
This wants backporting to stable releases, so I would recommend for 4.15
even at this point.
~Andrew
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