Re: [Qemu-devel] Default for phys-addr-bits? (was Re: [PATCH 4/5] x86: Allow physical address bits to be set)

[Paolo Bonzini]

On 22/06/2016 16:24, Andrea Arcangeli wrote:
> Linux could not possibly crash instead if host phys bits > guest phys
> bits because it will never depend on GPF triggering if the must be
> zero bits of the guest pagetables are set. Linux won't ever try to set
> those bits and I'd be shocked if any other OS does.

Well, KVM does.  It sets _all_ bits up to 51, not just one, but still we
have a counterexample.

The reason to do this is that you can distinguish a not-present from a
present-reserved page fault, and handle the present-reserved page fault
from a cache without having to walk the page tables.

Paolo

> So while not perfect emulation of the hardware, the risk with known OS
> should be zero.