Re: [Qemu-devel] [PATCH v6 1/3] IOMMU: add option to enable VTD_CAP_CM to vIOMMU capility exposoed to guest

[Jason Wang]

On 2016年11月11日 11:39, Michael S. Tsirkin wrote:
> On Fri, Nov 11, 2016 at 10:32:42AM +0800, Jason Wang wrote:
> > On 2016年11月10日 06:00, Michael S. Tsirkin wrote:
> > > On Wed, Nov 09, 2016 at 03:28:02PM +0800, Jason Wang wrote:
> > > > > On 2016年11月08日 19:04, Aviv B.D wrote:
> > > > > > > From: "Aviv Ben-David"<address@hidden>
> > > > > > >
> > > > > > > This capability asks the guest to invalidate cache before each map operation.
> > > > > > > We can use this invalidation to trap map operations in the hypervisor.
> > > > > Hi:
> > > > >
> > > > > Like I've asked twice in the past, I want to know why don't you cache
> > > > > translation faults as what spec required (especially this is a guest visible
> > > > > behavior)?
> > > > >
> > > > > Btw, please cc me on posting future versions.
> > > > >
> > > > > Thanks
> > > Caching isn't guest visible.
> > Seems not, if one fault mapping were cached by IOTLB. Guest can notice this
> > behavior.
> Sorry, I don't get what you are saying.
>
> > > Spec just says you*can*  cache,
> > > not that you must.
> > Yes, but what did in this patch is "don't". What I suggest is just a "can",
> > since anyway the IOTLB entries were limited and could be replaced by other.
> >
> > Thanks
> Have trouble understanding this. Can you given an example of
> a guest visible difference?

I guess this may do the detection:

1) map iova A to be non-present.
2) invalidate iova A
3) map iova A to addr B
4) access iova A

A correct implemented CM may meet fault in step 4, but with this patch, 
we never.