Re: [Qemu-devel] [PATCH 08/10] intel-iommu: maintain per-device iova ranges

[Jason Wang]

On 2018年05月03日 15:28, Peter Xu wrote:
> On Thu, May 03, 2018 at 03:20:11PM +0800, Jason Wang wrote:
> > On 2018年05月03日 14:04, Peter Xu wrote:
> > > IMHO the guest can't really detect this, but it'll found that the
> > > device is not working functionally if it's doing something like what
> > > Jason has mentioned.
> > >
> > > Actually now I have had an idea if we really want to live well even
> > > with Jason's example: maybe we'll need to identify PSI/DSI.  For DSI,
> > > we don't remap for mapped pages; for PSI, we unmap and remap the
> > > mapped pages.  That'll complicate the stuff a bit, but it should
> > > satisfy all the people.
> > >
> > > Thanks,
> > So it looks like there will be still unnecessary unamps.
> Could I ask what do you mean by "unecessary unmaps"?

It's for "for PSI, we unmap and remap the mapped pages". So for the 
first "unmap" how do you know it was really necessary without knowing 
the state of current shadow page table?

> > How about record the mappings in the tree too?
> As I mentioned, for L1 guest (e.g., DPDK applications running in L1)
> it'll be fine.  But I'm just afraid we will have other use cases, like
> the L2 guests. That might need tons of the mapping entries in the
> worst case scenario.

Yes, but that's the price of shadow page tables.

Thanks