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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

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.

It's basically equivalent to a physical system where the BIOS masks this
cpuid bit because of some hardware errata or something, the underlying
feature might appear to be present but will have more or less subtle
issues.

Ian.


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