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

[Peter Xu <peterx@redhat.com>]

On Thu, May 03, 2018 at 03:43:35PM +0800, Jason Wang wrote:
> 
> 
> 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?

I don't.  Could I just unmap it anyway?  Say, now the guest _modified_
the PTE already.  Yes I think it's following the spec, but it is
really _unsafe_.  We can know that from what it has done already.
Then I really think a unmap+map would be good enough for us...  After
all that behavior can cause DMA error even on real hardwares.  It can
never tell.

> 
> > > 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.

So that's why I would like to propose this mergable interval tree.  It
might greatly reduce the price if we can reach a consensus on how we
should treat those strange-behaved guest OSs.  Thanks,

-- 
Peter Xu