Re: [PATCH v2 11/17] x86/CPUID: adjust extended leaves out of range clearing

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 23/11/2020 14:32, Jan Beulich wrote:
> A maximum extended leaf input value with the high half different from
> 0x8000 should not be considered valid - all leaves should be cleared in
> this case.
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> v2: Integrate into series.
>
> --- a/tools/tests/cpu-policy/test-cpu-policy.c
> +++ b/tools/tests/cpu-policy/test-cpu-policy.c
> @@ -516,11 +516,22 @@ static void test_cpuid_out_of_range_clea
>              },
>          },
>          {
> +            .name = "no extd",
> +            .nr_markers = 0,
> +            .p = {
> +                /* Clears all markers. */
> +                .extd.max_leaf = 0,
> +
> +                .extd.vendor_ebx = 0xc2,
> +                .extd.raw_fms = 0xc2,
> +            },
> +        },
> +        {
>              .name = "extd",
>              .nr_markers = 1,
>              .p = {
>                  /* Retains marker in leaf 0.  Clears others. */
> -                .extd.max_leaf = 0,
> +                .extd.max_leaf = 0x80000000,
>                  .extd.vendor_ebx = 0xc2,
>  
>                  .extd.raw_fms = 0xc2,
> --- a/xen/lib/x86/cpuid.c
> +++ b/xen/lib/x86/cpuid.c
> @@ -232,7 +232,9 @@ void x86_cpuid_policy_clear_out_of_range
>                      ARRAY_SIZE(p->xstate.raw) - 1);
>      }
>  
> -    zero_leaves(p->extd.raw, (p->extd.max_leaf & 0xffff) + 1,
> +    zero_leaves(p->extd.raw,
> +                ((p->extd.max_leaf >> 16) == 0x8000
> +                 ? (p->extd.max_leaf & 0xffff) + 1 : 0),
>                  ARRAY_SIZE(p->extd.raw) - 1);

Honestly, this is unnecessary complexity and overhead, and the logic is
already hard enough to follow.

There won't be extd.max_leaf with the high half != 0x8000 in real
policies, because of how we fill them.  Nor ought there to be, given the
intended meaning of this part of the union.

I think we simply forbid this case, rather than taking extra complexity
to cope with it.  Approximately all VMs will have 0x80000008 as a
minimum, and I don't think catering to pre-64bit Intel CPUs is worth our
effort either.

~Andrew