linux-kernel - Re: [PATCHv5 0/8] Virtual NMI feature

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Message-ID: <fc8813c6-0091-8571-d934-e33d7d56123d@amd.com>
Date:   Wed, 16 Nov 2022 11:10:31 +0530
From:   Santosh Shukla <santosh.shukla@....com>
To:     Maxim Levitsky <mlevitsk@...hat.com>, pbonzini@...hat.com,
        seanjc@...gle.com, jmattson@...gle.com
Cc:     kvm@...r.kernel.org, joro@...tes.org, linux-kernel@...r.kernel.org,
        mail@...iej.szmigiero.name, thomas.lendacky@....com,
        vkuznets@...hat.com, santosh.shukla@....com
Subject: Re: [PATCHv5 0/8] Virtual NMI feature

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/

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
>