Re: [Xen-devel] [PATCH] libxc: Expose the pdpe1gb cpuid flag to guest

[Tim Deegan <tim@xxxxxxx>]

At 14:26 +0000 on 18 Nov (1416317209), Ian Campbell wrote:
> On Tue, 2014-11-18 at 11:41 +0000, Zhang, Yang Z wrote:
> > > Hmm - this is a pitfall waiting to happen.
> > > 
> > > In the case that there is a heterogeneous setup with one 1G capable
> > > and one 1G incapable server, Xen cannot forcibly prevent the use of 1G
> > > pages on the capable hardware.  Any VM which guesses at hardware
> > > support by means other than cpuid features is liable to explode on 
> > > migrate.
> > 
> > But a normal guest shouldn't to guess it, right?
> 
> IMHO any guest which does not use the mechanism explicitly provided for
> feature detection deserves to break randomly.

Yes, that's a reasonable position (notwithstanding that we know such
software exists).

In this case, the guest is entitled to _expect_ pagefaults on 1GB
mappings if CPUID claims they are not supported.  That sounds like an
unlikely thing for the guest to be relying on, but Xen itself does
something similar for the SHOPT_FAST_FAULT_PATH (and now also for
IOMMU entries for the deferred caching attribute updates).

Cheers,

Tim.

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