Re: [Xen-devel] [PATCH v3 11/28] xen/x86: Clear dependent features when clearing a cpu cap

[Konrad Rzeszutek Wilk <konrad@xxxxxxxxxx>]

On Tue, Mar 15, 2016 at 03:35:07PM +0000, Andrew Cooper wrote:
> When clearing a cpu cap, clear all dependent features.  This avoids having a
> featureset with intermediate features disabled, but leaf features enabled.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> ---
> CC: Jan Beulich <JBeulich@xxxxxxxx>
> 
> v3:
>  * Style fixes.  Use __test_and_set_bit()
> ---
>  xen/arch/x86/cpu/common.c | 16 +++++++++++++++-
>  1 file changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/xen/arch/x86/cpu/common.c b/xen/arch/x86/cpu/common.c
> index cd168c8..cb1cab3 100644
> --- a/xen/arch/x86/cpu/common.c
> +++ b/xen/arch/x86/cpu/common.c
> @@ -53,8 +53,22 @@ static unsigned int cleared_caps[NCAPINTS];
>  
>  void __init setup_clear_cpu_cap(unsigned int cap)
>  {
> +     const uint32_t *dfs;
> +     unsigned int i;
> +
> +     if (__test_and_set_bit(cap, cleared_caps))
> +             return;
> +
>       __clear_bit(cap, boot_cpu_data.x86_capability);
> -     __set_bit(cap, cleared_caps);
> +     dfs = lookup_deep_deps(cap);
> +
> +     if (!dfs)
> +             return;
> +
> +     for (i = 0; i < FSCAPINTS; ++i) {
> +             cleared_caps[i] |= dfs[i];
> +             boot_cpu_data.x86_capability[i] &= ~dfs[i];
> +     }

That is OK, but what if the feature we clear has a dependeny
chain - of clearing other features?
>  }
>  
>  static void default_init(struct cpuinfo_x86 * c)
> -- 
> 2.1.4
> 
> 
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