Re: [PATCH v2] x86/kvm: Disable KVM_ASYNC_PF_SEND_ALWAYS

[Andy Lutomirski <luto@amacapital.net>]

> On Apr 7, 2020, at 1:20 PM, Thomas Gleixner <tglx@linutronix.de> wrote:
> 
> Andy Lutomirski <luto@amacapital.net> writes:
>>>> On Apr 7, 2020, at 10:21 AM, Vivek Goyal <vgoyal@redhat.com> wrote:
>>> Whether interrupts are enabled or not check only happens before we decide
>>> if async pf protocol should be followed or not. Once we decide to
>>> send PAGE_NOT_PRESENT, later notification PAGE_READY does not check
>>> if interrupts are enabled or not. And it kind of makes sense otherwise
>>> guest process will wait infinitely to receive PAGE_READY.
>>> 
>>> I modified the code a bit to disable interrupt and wait 10 seconds (after
>>> getting PAGE_NOT_PRESENT message). And I noticed that error async pf
>>> got delivered after 10 seconds after enabling interrupts. So error
>>> async pf was not lost because interrupts were disabled.
> 
> Async PF is not the same as a real #PF. It just hijacked the #PF vector
> because someone thought this is a brilliant idea.
> 
>>> Havind said that, I thought disabling interrupts does not mask exceptions.
>>> So page fault exception should have been delivered even with interrupts
>>> disabled. Is that correct? May be there was no vm exit/entry during
>>> those 10 seconds and that's why.
> 
> No. Async PF is not a real exception. It has interrupt semantics and it
> can only be injected when the guest has interrupts enabled. It's bad
> design.
> 
>> My point is that the entire async pf is nonsense. There are two types of events right now:
>> 
>> “Page not ready”: normally this isn’t even visible to the guest — the
>> guest just waits. With async pf, the idea is to try to tell the guest
>> that a particular instruction would block and the guest should do
>> something else instead. Sending a normal exception is a poor design,
>> though: the guest may not expect this instruction to cause an
>> exception. I think KVM should try to deliver an *interrupt* and, if it
>> can’t, then just block the guest.
> 
> That's pretty much what it does, just that it runs this through #PF and
> has the checks for interrupts disabled - i.e can't right now' around
> that. If it can't then KVM schedules the guest out until the situation
> has been resolved.
> 
>> “Page ready”: this is a regular asynchronous notification just like,
>> say, a virtio completion. It should be an ordinary interrupt.  Some in
>> memory data structure should indicate which pages are ready.
>> 
>> “Page is malfunctioning” is tricky because you *must* deliver the
>> event. x86’s #MC is not exactly a masterpiece, but it does kind of
>> work.
> 
> Nooooo. This does not need #MC at all. Don't even think about it.

Yessssssssssss.  Please do think about it. :)

> 
> The point is that the access to such a page is either happening in user
> space or in kernel space with a proper exception table fixup.
> 
> That means a real #PF is perfectly fine. That can be injected any time
> and does not have the interrupt semantics of async PF.

The hypervisor has no way to distinguish between MOV-and-has-valid-stack-and-extable-entry and MOV-definitely-can’t-fault-here.  Or, for that matter, MOV-in-do_page_fault()-will-recurve-if-it-faults.

> 
> So now lets assume we distangled async PF from #PF and made it a regular
> interrupt, then the following situation still needs to be dealt with:
> 
>   guest -> access faults
> 
> host -> injects async fault
> 
>   guest -> handles and blocks the task
> 
> host figures out that the page does not exist anymore and now needs to
> fixup the situation.
> 
> host -> injects async wakeup
> 
>   guest -> returns from aysnc PF interrupt and retries the instruction
>            which faults again.
> 
> host -> knows by now that this is a real fault and injects a proper #PF
> 
>   guest -> #PF runs and either sends signal to user space or runs
>            the exception table fixup for a kernel fault.

Or guest blows up because the fault could not be recovered using #PF.

I can see two somewhat sane ways to make this work.

1. Access to bad memory results in an async-page-not-present, except that,  it’s not deliverable, the guest is killed. Either that async-page-not-present has a special flag saying “memory failure” or the eventual wakeup says “memory failure”.

2. Access to bad memory results in #MC.  Sure, #MC is a turd, but it’s an *architectural* turd. By all means, have a nice simple PV mechanism to tell the #MC code exactly what went wrong, but keep the overall flow the same as in the native case.

I think I like #2 much better. It has another nice effect: a good implementation will serve as a way to exercise the #MC code without needing to muck with EINJ or with whatever magic Tony uses. The average kernel developer does not have access to a box with testable memory failure reporting.

> 
> Thanks,
> 
>        tglx
> 
> 
> 
>