linux-kernel - Re: [PATCH 3/5] KVM: nSVM: Don't forget about L1-injected events

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Message-ID: <f4cdaf45-c869-f3bb-2ba2-3c0a4da12a6d@maciej.szmigiero.name>
Date:   Fri, 1 Apr 2022 01:09:48 +0200
From:   "Maciej S. Szmigiero" <mail@...iej.szmigiero.name>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Brijesh Singh <brijesh.singh@....com>,
        Jon Grimm <Jon.Grimm@....com>,
        David Kaplan <David.Kaplan@....com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Liam Merwick <liam.merwick@...cle.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/5] KVM: nSVM: Don't forget about L1-injected events

On 31.03.2022 01:20, Sean Christopherson wrote:
> On Thu, Mar 31, 2022, Maciej S. Szmigiero wrote:
>> On 30.03.2022 23:59, Sean Christopherson wrote:
>>> On Thu, Mar 10, 2022, Maciej S. Szmigiero wrote:
>>>> @@ -3627,6 +3632,14 @@ static void svm_complete_interrupts(struct kvm_vcpu *vcpu)
>>>>    	if (!(exitintinfo & SVM_EXITINTINFO_VALID))
>>>>    		return;
>>>> +	/* L1 -> L2 event re-injection needs a different handling */
>>>> +	if (is_guest_mode(vcpu) &&
>>>> +	    exit_during_event_injection(svm, svm->nested.ctl.event_inj,
>>>> +					svm->nested.ctl.event_inj_err)) {
>>>> +		nested_svm_maybe_reinject(vcpu);
>>>
>>> Why is this manually re-injecting?  More specifically, why does the below (out of
>>> sight in the diff) code that re-queues the exception/interrupt not work?  The
>>> re-queued event should be picked up by nested_save_pending_event_to_vmcb12() and
>>> propagatred to vmcb12.
>>
>> A L1 -> L2 injected event should either be re-injected until successfully
>> injected into L2 or propagated to VMCB12 if there is a nested VMEXIT
>> during its delivery.
>>
>> svm_complete_interrupts() does not do such re-injection in some cases
>> (soft interrupts, soft exceptions, #VC) - it is trying to resort to
>> emulation instead, which is incorrect in this case.
>>
>> I think it's better to split out this L1 -> L2 nested case to a
>> separate function in nested.c rather than to fill
>> svm_complete_interrupts() in already very large svm.c with "if" blocks
>> here and there.
> 
> Ah, I see it now.  WTF.
> 
> Ugh, commit 66fd3f7f901f ("KVM: Do not re-execute INTn instruction.") fixed VMX,
> but left SVM broken.
> 
> Re-executing the INTn is wrong, the instruction has already completed decode and
> execution.  E.g. if there's there's a code breakpoint on the INTn, rewinding will
> cause a spurious #DB.
> 
> KVM's INT3 shenanigans are bonkers, but I guess there's no better option given
> that the APM says "Software interrupts cannot be properly injected if the processor
> does not support the NextRIP field.".  What a mess.

Note that KVM currently always tries to re-execute the current instruction
when asked to re-inject a #BP or a #OF, even when nrips are enabled.

Also, #BP (and #OF, too) is returned as type SVM_EXITINTINFO_TYPE_EXEPT,
not as SVM_EXITINTINFO_TYPE_SOFT (soft interrupt), so it should be
re-injected accordingly.

> Anyways, for the common nrips=true case, I strongly prefer that we properly fix
> the non-nested case and re-inject software interrupts, which should in turn
> naturally fix this nested case.  

This would also need making the #BP or #OF current instruction
re-execution conditional on (at least) nrips support.

I am not sure, however, whether this won't introduce any regressions.
That's why this patch set changed the behavior here only for the
L1 -> L2 case.

Another issue is whether a L1 hypervisor can legally inject a #VC
into its L2 (since these are never re-injected).

We still need L1 -> L2 event injection detection to restore the NextRIP
field when re-injecting an event that uses it.

> And for nrips=false, my vote is to either punt
> and document it as a "KVM erratum", or straight up make nested require nrips.

A quick Internet search shows that the first CPUs with NextRIP were the
second-generation Family 10h CPUs (Phenom II, Athlon II, etc.).
They started being released in early 2009, so we probably don't need to
worry about the non-nrips case too much.

For the nested case, orthodox reading of the aforementioned APM sentence
would mean that a L1 hypervisor is not allowed either to make use of such
event injection in the non-nrips case.

> Note, that also requires updating svm_queue_exception(), which assumes it will
> only be handed hardware exceptions, i.e. hardcodes type EXEPT.  That's blatantly
> wrong, e.g. if userspace injects a software exception via KVM_SET_VCPU_EVENTS.

svm_queue_exception() uses SVM_EVTINJ_TYPE_EXEPT, which is correct even
for software exceptions (#BP or #OF).
It does work indeed, as the self test included in this patch set
demonstrates.

Thanks,
Maciej