Re: [Xen-devel] [PATCH v5 1/4] xen/libxc: Allow changes to hypervisor CPUID leaf from config file

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 19.03.14 at 01:58, Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx> wrote:
> Currently only "real" cpuid leaves can be overwritten by users via
> 'cpuid' option in the configuration file. This patch provides ability to
> do the same for hypervisor leaves (but for now only 0x40000000 is allowed).

I'm still not really happy with the approach:

> --- a/xen/arch/x86/traps.c
> +++ b/xen/arch/x86/traps.c
> @@ -690,6 +690,9 @@ int cpuid_hypervisor_leaves( uint32_t idx, uint32_t 
> sub_idx,
>      if ( idx > limit ) 
>          return 0;
>  
> +    if ( domain_cpuid_exists(d, base + idx, sub_idx, eax, ebx, ecx, edx) )
> +        return 1;

You basically rely here on the tools having fed back the values the
hypervisor returned to them. What if this gets tweaked at some
point based on some other domain characteristics? I'd really like to
see the hypervisor, and only the hypervisor, being in charge of
returning the hypervisor leaf values. The sole restriction the tools
may place ought to be on the maximum leaf.

Sadly I didn't see anyone else commenting on this model of yours,
so I can't judge whether I'm the only one having reservations
against the model you have been proposing so far.

Jan


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