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

[Thomas Gleixner]

Thomas Gleixner <tglx@xxxxxxxxxxxxx> writes:

> Thomas Gleixner <tglx@xxxxxxxxxxxxx> writes:
>> Andy Lutomirski <luto@xxxxxxxxxx> writes:
>>> On Sat, Mar 7, 2020 at 7:47 AM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>>>> The host knows exactly when it injects a async PF and it can store CR2
>>>> and reason of that async PF in flight.
>>>>
>>>> On the next VMEXIT it checks whether apf_reason is 0. If apf_reason is 0
>>>> then it knows that the guest has read CR2 and apf_reason. All good
>>>> nothing to worry about.
>>>>
>>>> If not it needs to be careful.
>>>>
>>>> As long as the apf_reason of the last async #PF is not cleared by the
>>>> guest no new async #PF can be injected. That's already correct because
>>>> in that case IF==0 which prevents a nested async #PF.
>>>>
>>>> If MCE, NMI trigger a real pagefault then the #PF injection needs to
>>>> clear apf_reason and set the correct CR2. When that #PF returns then the
>>>> old CR2 and apf_reason need to be restored.
>>>
>>> How is the host supposed to know when the #PF returns?  Intercepting
>>> IRET sounds like a bad idea and, in any case, is not actually a
>>> reliable indication that #PF returned.
>>
>> The host does not care about the IRET. It solely has to check whether
>> apf_reason is 0 or not. That way it knows that the guest has read CR2
>> and apf_reason.
>
> Bah. I'm a moron. Of course it needs to trap the IRET of the #PF in
> order to restore CR2 and apf_reason. Alternatively it could trap the CR2
> read of #PF, but yes that's all nasty.

Some hours or sleep and not staring at this meess later and while
reading the leaves of my morning tea:

guest side:

   nmi()/mce() ...
   
        stash_crs();

+       stash_and_clear_apf_reason();

        ....

+       restore_apf_reason();

	restore_cr2();

Too obvious, isn't it?

Thanks,

        tglx