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

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Date:   Wed, 16 Nov 2022 11:21:42 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Santosh Shukla <santosh.shukla@....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
Subject: Re: [PATCHv5 0/8] Virtual NMI feature

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).

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.

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
> > 
>