Vivek Goyal <vgoyal@xxxxxxxxxx> writes: > On Tue, Jun 30, 2020 at 03:24:43PM +0200, Vitaly Kuznetsov wrote: >> >> It's probably me who's missing something important here :-) but I think >> you describe how it *should* work as I'm not seeing how we can leave the >> loop in kvm_async_pf_task_wait_schedule() other than by >> "if (hlist_unhashed(&n.link)) break;" and this only happens when APF >> completes. > > We don't leave loop in kvm_async_pf_task_wait_schedule(). It will happen > before you return to user space. > > I have not looked too closely but I think following code path might be taken > after aync PF has completed. > > __kvm_handle_async_pf() > idtentry_exit_cond_rcu() > prepare_exit_to_usermode() > __prepare_exit_to_usermode() > exit_to_usermode_loop() > do_signal() > > So once you have been woken up (because APF completed), Ah, OK so we still need to complete APF and we can't kill the process before this happens, that's what I was missing. > you will > return to user space and before that you will check if there are > pending signals and handle that signal first before user space > gets a chance to run again and retry faulting instruction. ... > >> >> When guest receives the 'page ready' event with an error it (like for >> every other 'page ready' event) tries to wake up the corresponding >> process but if the process is dead already it can do in-kernel probing >> of the GFN, this way we guarantee that the error is always injected. I'm >> not sure if it is needed though but in case it is, this can be a >> solution. We can add a new feature bit and only deliver errors when the >> guest indicates that it knows what to do with them. > > - Process will be delivered singal after async PF completion and during > returning to user space. You have lost control by then. > So actually there's no way for kernel to know if the userspace process managed to re-try the instruction and get the error injected or if it was killed prior to that. > - If you retry in kernel, we will change the context completely that > who was trying to access the gfn in question. We want to retain > the real context and retain information who was trying to access > gfn in question. (Just so I understand the idea better) does the guest context matter to the host? Or, more specifically, are we going to do anything besides get_user_pages() which will actually analyze who triggered the access *in the guest*? -- Vitaly
