Re: [Xen-devel] [PATCH v2 1/3] Nested VMX: Check VMX capability before read VMX related MSRs.

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 10.09.13 at 08:14, Yang Zhang <yang.z.zhang@xxxxxxxxx> wrote:
> +    /*
> +     * Those MSRs available only when bit 55 of
> +     * MSR_IA32_VMX_BASIC is set.
> +     */
> +    rdmsrl(MSR_IA32_VMX_BASIC, data);
> +    if ( msr >= MSR_IA32_VMX_TRUE_PINBASED_CTLS &&
> +         msr <= MSR_IA32_VMX_TRUE_ENTRY_CTLS &&

Doing this with a range check is pretty odd - neither the lower nor
the upper bound are really meant to be boundaries. I'd prefer if
you did this with a switch statement, enumerating all four MSRs.
The compiler can then still convert that to a range comparison.

And that way you can also more the rdmsrl() out of the fast path.
Which makes me ask - wouldn't it be better to read this MSR into
a global variable once at boot time, and then use that variable
instead of reading the MSR over and over?

Jan

> +         !(data & VMX_BASIC_DEFAULT1_ZERO) )
> +        return 0;
> +



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