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

[Jason Wang]

On 2018年05月03日 15:53, Peter Xu wrote:
> 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.

I mean for following case:

1) guest maps A1 (iova) to XXX
2) guest maps A2 (A1 + 4K) (iova) to YYY
3) guest maps A3 (A1 + 8K) (iova) to ZZZ
4) guest unmaps A2 and A2, for reducing the number of PSIs, it can 
invalidate A1 with a range of 2M

If this is allowed by spec, looks like A1 will be unmaped and mapped.

Thanks

> > > > 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,