Re: [Xen-devel] [PATCH v12 3/9] tools: provide interface for generic MSR access

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 24.07.14 at 08:31, <dongxiao.xu@xxxxxxxxx> wrote:
> Considering many people in the list requires the white-list style to limit 
> the capability for resource access (e.g. MSR read/write), so I implement such 
> a white-list in my new version patch like following:
> Does it look reasonable to you?

Reasonable - perhaps. Efficiently coded - no:

> static unsigned int allowed_msr_list[] = {
>     MSR_IA32_QOSEVTSEL,
>     MSR_IA32_QMC,
> };
> 
> static unsigned int allow_access(unsigned int idx, unsigned int *list, 
> unsigned int nr)
> {
>     unsigned int i;
> 
>     for ( i = 0; i < nr; i++ )
>         if ( list[i] == idx )
>             return 1;

Using a big switch allows the compiler to (hopefully) find an optimal
translation - possibly via lookup table, but maybe via other means.
And if you were to stay with the explicit lookup table, you'd have
to fix various mechanical issues (but I take this only as a sketch,
not as something you're proposing as is).

That said I continue to not be fully convinced of the need of a white
or black list here, not the least with the consideration in mind of
extending this to port I/O.

Jan


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