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

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

On 2015/5/5 17:24, Jan Beulich wrote:
> > > > On 05.05.15 at 04:45, <tiejun.chen@xxxxxxxxx> wrote:
> > Does this work for everyone?
> Please first of all explain why the interfaces in asm/flushtlb.h can't
> be used here (at least when flushing entire pages). Because - as
I also don't understand any reason we didn't use this previously on 
IOMMU side...
> said before - for a complete fix you'd need to deal with the CLFLUSH
> use(s) elsewhere in the system too.
Looks we need to do this in xen/arch/x86/flushtlb.c:flush_area_local().

> Btw, isn't the !iommus_incoherent check in that function inverted?
> I.e. why would we _not_ need to flush caches when IOMMUs
> are not coherent (and why would flushing be needed when they're
> coherent anyway)?
I guess you're misunderstanding this

    if ( !ecap_coherent(iommu->ecap) )
        iommus_incoherent = 1;

So here !iommus_incoherent means IOMMU is coherent and then we don't 
need to flush cache in this case.
Thanks
Tiejun

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