Re: [Xen-devel] [PATCH 1/3] xen/vt-d: need barriers to workaround CLFLUSH

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 04.05.15 at 04:16, <tiejun.chen@xxxxxxxxx> wrote:
> --- a/xen/drivers/passthrough/vtd/x86/vtd.c
> +++ b/xen/drivers/passthrough/vtd/x86/vtd.c
> @@ -56,7 +56,9 @@ unsigned int get_cache_line_size(void)
>  
>  void cacheline_flush(char * addr)
>  {
> +    mb();
>      clflush(addr);
> +    mb();
>  }

I think the purpose of the flush is to force write back, not to evict
the cache line, and if so wmb() would appear to be sufficient. As
the SDM says that's not the case, a comment explaining why wmb()
is not sufficient would seem necessary. Plus in the description I
think "serializing" needs to be changed to "fencing", as serialization
is not what we really care about here. If you and the maintainers
agree, I could certainly fix up both aspects while committing.

Jan


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