On 08/04/20 15:01, Thomas Gleixner wrote:
>
> And it comes with restrictions:
>
> The Do Other Stuff event can only be delivered when guest IF=1.
>
> If guest IF=0 then the host has to suspend the guest until the
> situation is resolved.
>
> The 'Situation resolved' event must also wait for a guest IF=1 slot.
Additionally:
- the do other stuff event must be delivered to the same CPU that is
causing the host-side page fault
- the do other stuff event provides a token that identifies the cause
and the situation resolved event provides a matching token
This stuff is why I think the do other stuff event looks very much like
a #VE. But I think we're in violent agreement after all.
> If you just want to solve Viveks problem, then its good enough. I.e. the
> file truncation turns the EPT entries into #VE convertible entries and
> the guest #VE handler can figure it out. This one can be injected
> directly by the hardware, i.e. you don't need a VMEXIT.
>
> If you want the opportunistic do other stuff mechanism, then #VE has
> exactly the same problems as the existing async "PF". It's not magicaly
> making that go away.
You can inject #VE from the hypervisor too, with PV magic to distinguish
the two. However that's not necessarily a good idea because it makes it
harder to switch to hardware delivery in the future.
> One possible solution might be to make all recoverable EPT entries
> convertible and let the HW inject #VE for those.
>
> So the #VE handler in the guest would have to do:
>
> if (!recoverable()) {
> if (user_mode)
> send_signal();
> else if (!fixup_exception())
> die_hard();
> goto done;
> }
>
> store_ve_info_in_pv_page();
>
> if (!user_mode(regs) || !preemptible()) {
> hypercall_resolve_ept(can_continue = false);
> } else {
> init_completion();
> hypercall_resolve_ept(can_continue = true);
> wait_for_completion();
> }
>
> or something like that.
Yes, pretty much. The VE info can also be passed down to the hypercall
as arguments.
Paolo
> The hypercall to resolve the EPT fail on the host acts on the
> can_continue argument.
>
> If false, it suspends the guest vCPU and only returns when done.
>
> If true it kicks the resolve process and returns to the guest which
> suspends the task and tries to do something else.
>
> The wakeup side needs to be a regular interrupt and cannot go through
> #VE.
