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

[Michael S. Tsirkin]

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?