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
