Re: [PATCH 4.19] x86/mm/cpa: Unconditionally avoid WBINDV when we can

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Mon, Jul 04, 2022 at 11:45:08PM +0800, Wen Yang wrote:
> From: Peter Zijlstra <peterz@infradead.org>
> 
> commit ddd07b750382adc2b78fdfbec47af8a6e0d8ef37 upstream.
> 
> CAT has happened, WBINDV is bad (even before CAT blowing away the
> entire cache on a multi-core platform wasn't nice), try not to use it
> ever.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Reviewed-by: Dave Hansen <dave.hansen@intel.com>
> Cc: Bin Yang <bin.yang@intel.com>
> Cc: Mark Gross <mark.gross@intel.com>
> Link: https://lkml.kernel.org/r/20180919085947.933674526@infradead.org
> Cc: <stable@vger.kernel.org> # 4.19.x
> Signed-off-by: Wen Yang <wenyang@linux.alibaba.com>
> ---
>  arch/x86/mm/pageattr.c | 18 ++----------------
>  1 file changed, 2 insertions(+), 16 deletions(-)
> 
> diff --git a/arch/x86/mm/pageattr.c b/arch/x86/mm/pageattr.c
> index 101f3ad0d6ad..ab87da7a6043 100644
> --- a/arch/x86/mm/pageattr.c
> +++ b/arch/x86/mm/pageattr.c
> @@ -239,26 +239,12 @@ static void cpa_flush_array(unsigned long *start, int numpages, int cache,
>  			    int in_flags, struct page **pages)
>  {
>  	unsigned int i, level;
> -#ifdef CONFIG_PREEMPT
> -	/*
> -	 * Avoid wbinvd() because it causes latencies on all CPUs,
> -	 * regardless of any CPU isolation that may be in effect.
> -	 *
> -	 * This should be extended for CAT enabled systems independent of
> -	 * PREEMPT because wbinvd() does not respect the CAT partitions and
> -	 * this is exposed to unpriviledged users through the graphics
> -	 * subsystem.
> -	 */
> -	unsigned long do_wbinvd = 0;
> -#else
> -	unsigned long do_wbinvd = cache && numpages >= 1024; /* 4M threshold */
> -#endif
>  
>  	BUG_ON(irqs_disabled() && !early_boot_irqs_disabled);
>  
> -	on_each_cpu(__cpa_flush_all, (void *) do_wbinvd, 1);
> +	flush_tlb_all();
>  
> -	if (!cache || do_wbinvd)
> +	if (!cache)
>  		return;
>  
>  	/*
> -- 
> 2.19.1.6.gb485710b
> 

Why is this needed on 4.19.y?  What problem does it solve, it looks only
like an optimization, not a bugfix.

And if it's a bugfix, why only 4.19.y, why not older kernels too?

We need more information here please.

thanks,

greg k-h