Re: [PATCHv5 0/8] Virtual NMI feature

[Santosh Shukla]

Hello Maxim,.

On 11/16/2022 2:51 PM, Maxim Levitsky wrote:
> On Wed, 2022-11-16 at 11:10 +0530, Santosh Shukla wrote:
>> Hi Maxim,
>>
>> On 11/14/2022 8:01 PM, Maxim Levitsky wrote:
>>> On Mon, 2022-11-14 at 13:32 +0530, Santosh Shukla wrote:
>>>>
>>>>
>>>> On 10/27/2022 2:08 PM, Santosh Shukla wrote:
>>>>> VNMI Spec is at [1].
>>>>>
>>>>> Change History:
>>>>>
>>>>> v5 (6.1-rc2)
>>>>> 01,02,06 - Renamed s/X86_FEATURE_V_NMI/X86_FEATURE_AMD_VNMI (Jim Mattson)
>>>>>
>>>>
>>>> Gentle reminder.
>>>>
>>>> Thanks,
>>>> Santosh
>>>>
>>>
>>> I started reviewing it today and I think there are still few issues,
>>> and the biggest one is that if a NMI arrives while vNMI injection
>>> is pending, current code just drops such NMI.
>>>
>>> We had a discussion about this, like forcing immeditate vm exit
>>
>> I believe, We discussed above case in [1] i.e.. HW can handle
>> the second (/pending)virtual NMI while the guest processing first virtual NMI w/o vmexit.
>> is it same scenario or different one that you are mentioning?
>>
>> [1] https://lore.kernel.org/lkml/1782cdbb-8274-8c3d-fa98-29147f1e5d1e@xxxxxxx/>> 
> You misunderstood the issue.
> 
> Hardware can handle the case when a NMI is in service (that is V_NMI_MASK is set) and another one is injected 
> (V_NMI_PENDING can be set),
> 
> but it is not possible to handle the case when a NMI is already injected (V_NMI_PENDING set) but
> and KVM wants to inject another one before the first one went into the service (that is V_NMI_MASK is not set
> yet).
> 

In this case, HW will collapse the NMI.

Note that the HW will take the pending NMI at the boundary of IRET instruction such that
it will check for the V_NMI_PENDING and if its set then HW will *take* the NMI,
HW will clear the V_NMI_PENDING bit and set the V_NMI_MASK w/o the VMEXIT!,.


> Also same can happen when NMIs are blocked in SMM, since V_NMI_MASK is set despite no NMI in service,
> we will be able to inject only one NMI by setting the V_NMI_PENDING.
>

Ditto,. HW will collapse the NMI.

Thanks,
Santosh
 
> I think I was able to solve all these issues and I will today post a modified patch series of yours,
> which should cover all these cases and have some nice refactoring as well.
> 
> 
> Best regards,
> 	Maxim Levitsky
> 
> 
>>
>> Thanks,
>> Santosh
>>
>>> in this case and such but I have a simplier idea:
>>>
>>> In this case we can just open the NMI window in the good old way
>>> by intercepting IRET, STGI, and or RSM (which is intercepted anyway),
>>>
>>> and only if we already *just* intercepted IRET, only then just drop 
>>> the new NMI instead of single stepping over it based on reasoning that
>>> its 3rd NMI (one is almost done the servicing (its IRET is executing),
>>> one is pending injection, and we want to inject another one.
>>>
>>> Does this sound good to you? It won't work for SEV-ES as it looks
>>> like it doesn't intercept IRET, but it might be a reasonable tradeof
>>> for SEV-ES guests to accept that we can't inject a NMI if one is
>>> already pending injection.
>>>
>>> Best regards,
>>>         Maxim Levitsky
>>>
>>
> 
>