On Thu, May 03, 2018 at 05:22:03PM +0800, Jason Wang wrote: > > > 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.
My follow-up patch won't survive with this one but the original patch will work. Jason and I discussed a bit on IRC on this matter. Here's the conclusion we got: for now we use my original patch (which solves everything except PTE modifications). We mark that modify-PTE problem as TODO. Then at least we can have the nested device assignment work well on known OSs first. -- Peter Xu
