Re: [Xen-devel] [PATCH v10 16/16] microcode/intel: writeback and invalidate cache conditionally

[Jan Beulich <jbeulich@xxxxxxxx>]

On 12.09.2019 09:22, Chao Gao wrote:
> --- a/xen/arch/x86/microcode_intel.c
> +++ b/xen/arch/x86/microcode_intel.c
> @@ -305,6 +305,31 @@ static bool is_blacklisted(void)
>      return false;
>  }
>  
> +static void microcode_quirk(void)
> +{
> +    struct cpuinfo_x86 *c;

const

> +    uint64_t llc_size;
> +
> +    /*
> +     * Don't refer to current_cpu_data, which isn't fully initialized
> +     * before this stage.
> +     */
> +    if ( system_state < SYS_STATE_smp_boot )
> +        return;

If the workaround is needed, why would it not be needed for the BSP?

> +    c = &current_cpu_data;
> +    llc_size = c->x86_cache_size * 1024ULL;
> +    do_div(llc_size, c->x86_max_cores);

Instead of the local variable, ...

> +
> +    /*
> +     * To mitigate some issues on this specific Broadwell CPU, writeback and
> +     * invalidate cache regardless of ucode revision.
> +     */
> +    if ( c->x86 == 6 && c->x86_model == 0x4F && c->x86_mask == 0x1 &&
> +         llc_size > 2621440 )

... why don't you compare c->x86_cache_size / c->x86_max_cores
against 2560 here? Is there any risk of truncating relevant low
bits?

Jan

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