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21日 20:41, Aviv B.D. wrote:
> On Fri, Nov 11, 2016 at 6:15 AM, Jason Wang <address@hidden 
> <mailto:address@hidden>> wrote:
>
>
>
>     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
>                                 <mailto: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.
>
>
> By the specification (from intel) section 6.1 (Caching mode) tells 
> that this bit may be present
> only on virtual machines.

Yes, but we should also align the behavior to the spec.

> So with just this point the guest can detect that it running in
> a VM of some sort. Additionally, the wording of the specification 
> enable the host to issue a fault
> it your scenario but, doesn't requiring it.

I'm not sure I get the meaning, is there a section that describes your 
meaning in the spec?

Thanks

> Thanks,
> Aviv.