shared features in cpuid

shared features in cpuid

[Olaf Hering]

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While reviewing changed behavior in 'xen-cpuid -d' output between Xen 4.8 and 4.9 I found commit 20e92c97f904aa460e2223c30dcad36c234496b6 ("x86/cpuid: Only recalculate the shared feature bits once").

I wonder what the mentioned "cross-vendor case" in the comment, which was removed from sanitise_featureset(), really means.

Does Xen attempt to support domU migration between AMD and Intel hardware?


Olaf

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Re: shared features in cpuid

[Andrew Cooper]

On 21/08/2020 11:12, Olaf Hering wrote:
> While reviewing changed behavior in 'xen-cpuid -d' output between Xen 4.8 and 4.9 I found commit 20e92c97f904aa460e2223c30dcad36c234496b6 ("x86/cpuid: Only recalculate the shared feature bits once").
>
> I wonder what the mentioned "cross-vendor case" in the comment, which was removed from sanitise_featureset(), really means.
>
> Does Xen attempt to support domU migration between AMD and Intel hardware?

http://developer.amd.com/wordpress/media/2012/10/CrossVendorMigration.pdf

This was a feature put in a decade ago.

In practice, cross vendor migration works fine for PV guests, because
almost all of the vendor differences are abstracted away by the PV ABI.

We have no idea if anyone uses it still, but there is quite a lot of
ancillary code for it which we try our best not to break.

~Andrew