Re: [patch]pageattr: cache flush before tlb flush

["H. Peter Anvin" <hpa@zytor.com>]

Shaohua Li wrote:
> clflush uses a virtual address but cache line is physical indexed in
> x86. In my understanding, clflush will do some pagetable walk, so doing
> cache flush first should reduce some pagetable walk.
> 
> Signed-off-by: Shaohua Li <shaohua.li@intel.com>

I would say NAK on this.

Doing the CLFLUSH first does cut down on page table walking, but opens a 
hole in the sequencing: first set PAT to an uncachable mode, then flush.

If an unlucky prefetch comes in during this window, then you will have a 
dirty cache again.

So no, this is not a good idea.

	-hpa