Re: [PATCHv5 0/8] Virtual NMI feature

Tom Lendacky
2022-11-16T15:56:26Z

On 11/16/22 09:44, Santosh Shukla wrote:
> 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@amd.com/>>
>> 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.

Note, this is how bare-metal NMIs are also handled. Multiple NMIs are
collapsed into a single NMI if they are received while an NMI is currently
being processed.

Thanks,
Tom

>
> 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
>>>>
>>>
>>
>>
>