linux-kernel - Re: [PATCH kernel v5 5/6] KVM: SEV: Enable data breakpoints in SEV-ES

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Date:   Fri, 2 Jun 2023 09:31:05 +1000
From:   Alexey Kardashevskiy <aik@....com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     kvm@...r.kernel.org, x86@...nel.org, linux-kernel@...r.kernel.org,
        Tom Lendacky <thomas.lendacky@....com>,
        Pankaj Gupta <pankaj.gupta@....com>,
        Nikunj A Dadhania <nikunj@....com>,
        Santosh Shukla <santosh.shukla@....com>,
        Carlos Bilbao <carlos.bilbao@....com>
Subject: Re: [PATCH kernel v5 5/6] KVM: SEV: Enable data breakpoints in SEV-ES

Sean, ping?

I wonder if this sev-es-not-singlestepping is a showstopper or it is 
alright to repost this patchset without it? Thanks,


On 30/5/23 18:57, Alexey Kardashevskiy wrote:
> 
> 
> On 27/5/23 00:39, Sean Christopherson wrote:
>> On Fri, May 26, 2023, Alexey Kardashevskiy wrote:
>>>
>>> On 24/5/23 01:44, Sean Christopherson wrote:
>>>> On Tue, May 23, 2023, Alexey Kardashevskiy wrote:
>>>>>> Actually, can't disabling #DB interception for DebugSwap SEV-ES 
>>>>>> guests be a
>>>>>> separate patch?  KVM can still inject #DBs for SEV-ES guests, no?
>>>>>
>>>>> Sorry for my ignorance but what is the point of injecting #DB if 
>>>>> there is no
>>>>> way of changing the guest's DR7?
>>>>
>>>> Well, _injecting_ the #DB is necessary for correctness from the 
>>>> guest's perspective.
>>>> "What's the point of _intercepting_ #DB" is the real question.  And 
>>>> for SEV-ES guests
>>>> with DebugSwap, there is no point, which is why I agree that KVM 
>>>> should disable
>>>> interception in that case.  What I'm calling out is that disabling 
>>>> #Db interception
>>>> isn't _necessary_ for correctness (unless I'm missing something), 
>>>> which means that
>>>> it can and should go in a separate patch.
>>>
>>>
>>> About this. Instead of sev_es_init_vmcb(), I can toggle the #DB 
>>> intercept
>>> when toggling guest_debug, see below. This
>>> kvm_x86_ops::update_exception_bitmap hook is called on vcpu reset and
>>> kvm_arch_vcpu_ioctl_set_guest_debug (which skips this call if
>>> guest_state_protected = true).
>>
>> KVM also intercepts #DB when single-stepping over IRET to find an NMI 
>> window, so
>> you'd also have to factor in nmi_singlestep, and update 
>> svm_enable_nmi_window()
>> and disable_nmi_singlestep() to call svm_update_exception_bitmap().
> 
> Uff. New can of worms for me :-/
> 
> 
>>> Is there any downside?
>>
>> Complexity is the main one.  The complexity is quite low, but the 
>> benefit to the
>> guest is likely even lower.  A #DB in the guest isn't likely to be 
>> performance
>> sensitive.  And on the flip side, opening an NMI window would be a 
>> tiny bit more
>> expensive, though I doubt that would be meaningful either.
>>
>> All in all, I think it makes sense to just keep intercepting #DB for 
>> non-SEV-ES
>> guests.
>>
>> Side topic, isn't there an existing bug regarding SEV-ES NMI windows?  
>> KVM can't
>> actually single-step an SEV-ES guest, but tries to set RFLAGS.TF anyways. 
> 
> Why is it a "bug" and what does the patch fix? Sound to me as it is 
> pointless and the guest won't do single stepping and instead will run 
> till it exits somehow, what do I miss?
> 
>> Blech,
>> and suppressing EFER.SVME in efer_trap() is a bit gross,
> 
> Why suppressed? svm_set_efer() sets it eventually anyway.
> 
>> but I suppose since the
>> GHCB doesn't allow for CLGI or STGI it's "fine".
> 
> GHCB does not mention this, instead these are always intercepted in 
> init_vmcb().
> 
>> E.g. shouldn't KVM do this?
> 
> It sure can and I am happy to include this into the series, the commit 
> log is what I am struggling with :)
> 
>>
>> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
>> index ca32389f3c36..4e4a49031efe 100644
>> --- a/arch/x86/kvm/svm/svm.c
>> +++ b/arch/x86/kvm/svm/svm.c
>> @@ -3784,6 +3784,16 @@ static void svm_enable_nmi_window(struct 
>> kvm_vcpu *vcpu)
>>          if (svm_get_nmi_mask(vcpu) && !svm->awaiting_iret_completion)
>>                  return; /* IRET will cause a vm exit */
>> +       /*
>> +        * KVM can't single-step SEV-ES guests and instead assumes 
>> that IRET
>> +        * in the guest will always succeed, 
> 
> It relies on GHCB's NMI_COMPLETE (which SVM than handles is it was IRET):
> 
>          case SVM_VMGEXIT_NMI_COMPLETE:
>                  ret = svm_invoke_exit_handler(vcpu, SVM_EXIT_IRET);
>                  break;
> 
> 
>> i.e. clears NMI masking on the
>> +        * next VM-Exit.  Note, GIF is guaranteed to be '1' for SEV-ES 
>> guests
>> +        * as the GHCB doesn't allow for CLGI or STGI (and KVM suppresses
>> +        * EFER.SVME for good measure, see efer_trap()).
> 
> SVM KVM seems to not enforce EFER.SVME, the guest does what it wants and 
> KVM is only told the new value via EFER_WRITE_TRAP. And "writes by 
> SEV-ES guests to EFER.SVME are always ignored by hardware" says the APM. 
> I must be missing the point here...
> 
> 
>> +        */
>> +       if (sev_es_guest(vcpu->kvm))
>> +               return;
>> +
>>          if (!gif_set(svm)) {
>>                  if (vgif)
>>                          svm_set_intercept(svm, INTERCEPT_STGI);
> 

-- 
Alexey