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

["Chen, Tiejun" <tiejun.chen@xxxxxxxxx>]

On 2015/5/4 16:52, Jan Beulich wrote:
> > > > 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
Seems wmb() is not sufficient here.

"CLFLUSH is only ordered by the MFENCE instruction. It is not guaranteed 
to be ordered by any other fencing, serializing or other CLFLUSH 
instruction."
Thanks
Tiejun

> 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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